Followers of the Houthi movement raise their rifles as they shout slogans during a rally against the Saudi-led coalition in Yemen's capital Sanaa, August 11, 2015. Militiamen clashed with Yemen's dominant Houthi group inside the city of Ibb on Tuesday, residents said, in their latest significant advance into territory the Houthis had held unopposed for months. REUTERS/Mohamed al-Sayaghi 

Followers of the Houthi movement raise huge Yemeni flags during a rally against the Saudi-led coalition in Yemen's capital Sanaa, August 11, 2015. Militiamen clashed with Yemen's dominant Houthi group inside the city of Ibb on Tuesday, residents said, in their latest significant advance into territory the Houthis had held unopposed for months. REUTERS/Mohamed al-Sayaghi 

A boy holds his father's rifle during a rally by followers of the Houthi movement against the Saudi-led coalition in Yemen's capital Sanaa, August 11, 2015. Militiamen clashed with Yemen's dominant Houthi group inside the city of Ibb on Tuesday, residents said, in their latest significant advance into territory the Houthis had held unopposed for months. REUTERS/Mohamed al-Sayaghi 

A mountain tourist resort is seen one day after Sauid-led air strikes hit it in Yemen's central city of Ibb May 13, 2015. A five-day humanitarian truce in Yemen appeared to be broadly holding on Wednesday, despite reports of some air strikes overnight by Saudi-led forces and continued actions by the country's dominant Houthi group in the east. REUTERS/Stringer 

Fighters of the anti-Houthi Popular Resistance Committee secure a highway road linking Yemen's capital Sanaa with southern provinces May 5, 2015. REUTERS/Stringer 

A man walks past a bridge after it was hit by an air strike in Yemen's central province of Ibb April 21, 2015. REUTERS/Stringer 

A stadium is seen after it was destroyed by an air strike in Yemen's central city of Ibb April 13, 2015. REUTERS/Stringer 

A two-storey building is seen after it was destroyed by an air strike in Yemen's central city of Ibb April 13, 2015. REUTERS/Stringer 

Smoke billows after an air strike in Yemen's central city of Ibb April 12, 2015. REUTERS/Stringer 

Smoke billows after an air strike in Yemen's central city of Ibb April 12, 2015. REUTERS/Stringer 

People gather at the site of an air strike in Yareem city of Yemen's central province of Ibb March 31, 2015. Air raids by a Saudi-led coalition again hit Houthi militia targets across Yemen on Monday night, striking the group's northern stronghold of Saadeh, the capital, Sanaa, and the central town of Yarim, residents and media said. REUTERS/Stringer 

People gather at the site of an air strike in Yareem city of Yemen's central province of Ibb March 31, 2015. Air raids by a Saudi-led coalition again hit Houthi militia targets across Yemen on Monday night, striking the group's northern stronghold of Saadeh, the capital, Sanaa, and the central town of Yarim, residents and media said. REUTERS/Stringer 

People gather at the site of an air strike in Yareem city of Yemen's central province of Ibb March 31, 2015. Air raids by a Saudi-led coalition again hit Houthi militia targets across Yemen on Monday night, striking the group's northern stronghold of Saadeh, the capital, Sanaa, and the central town of Yarim, residents and media said. REUTERS/Stringer 

A boy walks at the site of an air strike in Yareem city of Yemen's central province of Ibb March 31, 2015. Air raids by a Saudi-led coalition again hit Houthi militia targets across Yemen on Monday night, striking the group's northern stronghold of Saadeh, the capital, Sanaa, and the central town of Yarim, residents and media said. REUTERS/Stringer 

Anti-Houthi protesters demonstrate to show support for Yemen's President Abdu-Rabbu Mansour Hadi in the central city of Ibb March 21, 2015. Hadi on Saturday accused the Iranian-allied Houthi militia that controls the capital Sanaa of staging a coup against him, and said he would "raise Yemen's flag" in the Houthis' northern stronghold. The banner (R) reads: "No to sectarianism... No to the civil war." REUTERS/Mohammed al-Moailme 

Anti-Houthi protesters demonstrate to commemorate the anniversary of an attack on pro-democracy protesters, in Yemen's central city of Ibb March 18, 2015. The attack, known as the "Friday of Dignity Massacre", on protesters rallying in Sanaa against the rule of Yemen's former President Ali Abdullah Saleh, left scores dead in March 2011. The banner reads: "The Friday of Dignity is a live memory for the nation that doesn't die." REUTERS/Stringer (YEMEN - Tags: CIVIL UNREST POLITICS) 

Anti-Houthi protesters demonstrate in Yemen's central city of Ibb March 14, 2015. The rise to power of the Iran-backed Houthis since September has deepened divisions in Yemen's already complex web of political and religious allegiances, and left it increasingly cut off from the world. REUTERS/Mohammed al-Moailme (YEMEN - Tags: CIVIL UNREST POLITICS) 

Anti-Houthi protesters demonstrate in Yemen's central city of Ibb March 14, 2015. The rise to power of the Iran-backed Houthis since September has deepened divisions in Yemen's already complex web of political and religious allegiances, and left it increasingly cut off from the world. REUTERS/Mohammed al-Moailme (YEMEN - Tags: CIVIL UNREST POLITICS) 

People march during an anti-Houthi demonstration in Yemen's central city of Ibb March 7, 2015. Yemen has plunged deeper into turmoil since the Iranian-backed Shi'ite Muslim Houthi group seized control of the capital in September last year. REUTERS/Mohammed al-Moailme (YEMEN - Tags: POLITICS CIVIL UNREST) 

Women march during an anti-Houthi demonstration in Yemen's central city of Ibb March 7, 2015. Yemen has plunged deeper into turmoil since the Iranian-backed Shi'ite Muslim Houthi group seized control of the capital in September last year. REUTERS/Mohammed al-Moailme (YEMEN - Tags: POLITICS CIVIL UNREST) 

Women take part in an anti-Houthi demonstration in Yemen's central city of Ibb March 7, 2015. Yemen has plunged deeper into turmoil since the Iranian-backed Shi'ite Muslim Houthi group seized control of the capital in September last year. The banner (top) reading "Freedom for the kidnapped" refers to activists detained by the Houthi movement. REUTERS/Mohammed al-Moailme (YEMEN - Tags: POLITICS CIVIL UNREST) 

People march during an anti-Houthi demonstration in Yemen's central city of Ibb March 7, 2015. Yemen has plunged deeper into turmoil since the Iranian-backed Shi'ite Muslim Houthi group seized control of the capital in September last year. REUTERS/Mohammed al-Moailme (YEMEN - Tags: POLITICS CIVIL UNREST) 

People perform the weekly Friday prayers during an anti-Houthi rally in Yemen's central city of Ibb March 6, 2015. Yemen has plunged deeper into turmoil since the Iranian-backed Shi'ite Muslim Houthi group seized control of the capital in September last year. REUTERS/Mohammed al-Moailme (YEMEN - Tags: CIVIL UNREST POLITICS RELIGION) 

Anti-Houthi protesters march during a demonstration to show support to Yemen's President Abd-Rabbu Mansour Hadi in the central city of Ibb February 28, 2015. Yemen's capital Sanaa was captured in September by the Shi'ite Muslim Houthi militia, which placed Hadi under house arrest and forced him to announce his resignation. Arab and Western states this month evacuated their Sanaa embassies. Parliament never approved the resignation, and on Sunday Hadi fled to Aden where he has set up a new seat of power. REUTERS/Mohammed al-Moailme (YEMEN - Tags: CIVIL UNREST POLITICS) 

Anti-Houthi protesters march during a demonstration to show support to Yemen's President Abd-Rabbu Mansour Hadi in the central city of Ibb February 28, 2015. Yemen's capital Sanaa was captured in September by the Shi'ite Muslim Houthi militia, which placed Hadi under house arrest and forced him to announce his resignation. Arab and Western states this month evacuated their Sanaa embassies. Parliament never approved the resignation, and on Sunday Hadi fled to Aden where he has set up a new seat of power. REUTERS/Essam al-Kamaly (YEMEN - Tags: CIVIL UNREST POLITICS) 

Protesters shout slogans during a demonstration to show support for Yemen's President Abd-Rabbu Mansour Hadi in the central city of Ibb February 27, 2015. Hadi fled to Aden last week after the newly dominant Shi'ite Houthi faction forced him to step down and held him under house arrest in Sanaa for a month. REUTERS/Mohammed al-Moailme (YEMEN - Tags: CIVIL UNREST POLITICS) 

Protesters shout slogans during a demonstration in support of Yemen's President Abd-Rabbu Mansour Hadi, in Yemen's central town of Ibb February 23, 2015. Hadi sought to resume his duties as head of state on Sunday, holding his first public engagement with state officials since he fled house arrest in Sanaa by the Houthi group that dominates the northern half of the country. REUTERS/Essam al-Kamaly (YEMEN - Tags: POLITICS CIVIL UNREST) 

Protesters shout slogans during a demonstration in support of Yemen's President Abd-Rabbu Mansour Hadi, in Yemen's central town of Ibb February 23, 2015. Hadi sought to resume his duties as head of state on Sunday, holding his first public engagement with state officials since he fled house arrest in Sanaa by the Houthi group that dominates the northern half of the country. REUTERS/Essam al-Kamaly (YEMEN - Tags: POLITICS CIVIL UNREST) 

Armed Houthis gather around the wreckage of a vehicle set a blaze by angry protesters after Houthis fired on them in Yemen's central town of Ibb February 14, 2015. Tens of thousands of Yemenis demonstrated in several cities on Saturday against the rule of the Shi'ite Muslim Houthi movement whose gunmen fired on protesters in the central town of Ibb and wounded four, medics said. REUTERS/Essam al-Kamaly (YEMEN - Tags: CIVIL UNREST POLITICS) 

Debris and shoes are seen at a cultural centre following a suicide attack in Ibb, central Yemen December 31, 2014. A suicide bomber killed at least 33 people in Ibb on Wednesday when he blew himself up at the cultural centre where students were celebrating Prophet Mohammad's birthday, medical sources and a resident said. REUTERS/Stringer (YEMEN - Tags: CIVIL UNREST) 

A soldier is seen outside a cultural centre following a suicide attack in Ibb, central Yemen December 31, 2014. A suicide bomber killed at least 33 people in Ibb on Wednesday when he blew himself up at the cultural centre where students were celebrating Prophet Mohammad's birthday, medical sources and a resident said. REUTERS/Stringer (YEMEN - Tags: CIVIL UNREST) 

Followers of the Shi'ite Houthi group prepare to lower the coffin of a man killed by al Qaeda militants in Yemen's central city of Ibb, during a funeral procession in Sanaa November 30, 2014. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: CIVIL UNREST POLITICS) 

Followers of the Shi'ite Houthi group prepare to lower the coffin of a man killed by al Qaeda militants in Yemen's central city of Ibb, during a funeral procession in Sanaa November 30, 2014. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: CIVIL UNREST POLITICS) 

Followers of the Shi'ite Houthi group carry the coffin of a man killed by al Qaeda militants in Yemen's central city of Ibb, during a funeral procession in Sanaa November 30, 2014. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: CIVIL UNREST POLITICS) 

Followers of the Shi'ite Houthi group carry the coffin of a man killed by al Qaeda militants in Yemen's central city of Ibb, during a funeral procession in Sanaa November 30, 2014. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: CIVIL UNREST POLITICS) 

A view of the historic town of Jibla in Yemen's central province of Ibb October 7, 2013. Jibla and its surroundings have been added to the UNESCO World Heritage tentative list due to its purported universal cultural value. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of houses at the historic town of Jibla in Yemen's central province of Ibb October 7, 2013. Jibla and its surroundings have been added to the UNESCO World Heritage tentative list due to its purported universal cultural value. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the historic town of Jibla in Yemen's central province of Ibb October 7, 2013. Jibla and its surroundings have been added to the UNESCO World Heritage tentative list due to its purported universal cultural value. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

Women walk past houses at the historic town of Jibla in Yemen's central province of Ibb October 7, 2013. Jibla and its surroundings have been added to the UNESCO World Heritage tentative list due to its purported universal cultural value. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the historic town of Jibla in Yemen's central province of Ibb October 7, 2013. Jibla and its surroundings have been added to the UNESCO World Heritage tentative list due to its purported universal cultural value. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

Girls look from a window of their house at historic town of Jibla in Yemen's central province of Ibb October 7, 2013. Jibla and its surroundings have been added to the UNESCO World Heritage tentative list due to its purported universal cultural value. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

People chat as they sit next to a closed shop in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY) 

A man walks as another prays at the grand mosque in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE RELIGION) 

Men sit next to closed shops in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY) 

A man prays at a mosque in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY RELIGION CITYSCAPE) 

A man sits next to closed shops in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY) 

A man walks in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY) 

A woman sits in an alley in the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY) 

A view of the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the Old Ibb city, the provincial capital of Yemen's central province of Ibb October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the Old Ibb city, the provincial capital of Yemen's central province of Ibb October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the Old Ibb city, the provincial capital of Yemen's central province of Ibb October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the Old Ibb city, the provincial capital of Yemen's central province of Ibb October 6, 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: SOCIETY CITYSCAPE) 

A view of the Old City of Ibb, the provincial capital of Yemen's central province of Ibb, October 6 2013. Yemeni authorities are seeking to add the historic city to the World Heritage List of the United Nations Educational, Scientific and Cultural Organization (UNESCO), media reports said. REUTERS/Khaled Abdullah (YEMEN - Tags: CITYSCAPE SOCIETY) 

Somali and Ethiopian refugees drink water from a well after arriving at the coasts of al-Mokha on their way to the cities of Taiz and Ibb, in the southern Yemeni province of Taiz March 29, 2013. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: SOCIETY IMMIGRATION) 

Somali and Ethiopian refugees arrive at the coasts of al-Mokha on their way to the cities of Taiz and Ibb, in the southern Yemeni province of Taiz March 29, 2013. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: SOCIETY IMMIGRATION) 

Somali and Ethiopian refugees walk after arriving at the coasts of al-Mokha on their way to the cities of Taiz and Ibb, in the southern Yemeni province of Taiz March 29, 2013. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: SOCIETY IMMIGRATION) 

Somali and Ethiopian refugees arrive at the coasts of al-Mokha on their way to the cities of Taiz and Ibb, in the southern Yemeni province of Taiz March 29, 2013. REUTERS/Mohamed al-Sayaghi (YEMEN - Tags: SOCIETY IMMIGRATION)