Men carry an injured civilian at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Maaret al-Naaman town in Idlib province June 4, 2015. REUTERS/Khalil Ashawi TPX IMAGES OF THE DAY

Men carry an injured man at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Maaret al-Naaman town in Idlib province June 4, 2015. REUTERS/Khalil Ashawi 

A man inspects a damaged building at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Maaret al-Naaman town in Idlib province June 4, 2015. REUTERS/Khalil Ashawi 

A man holds a baby that survived what activists said was a site hit by a barrel bomb dropped by forces loyal to Syrian President Bashar al-Assad at the old city of Aleppo June 3, 2015. REUTERS/Abdalrhman Ismail TPX IMAGES OF THE DAY

A Civil Defence member carrying a gas mask stands amid debris near a fire caused by what activists said was a barrel bomb dropped by forces loyal to Syria's president Bashar Al-Assad in Aleppo's al-Saliheen district, May 1, 2015. REUTERS/Hosam Katan 

A damaged car is seen at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's district of al-Sukari March 16, 2014. REUTERS/Ammar Abdullah (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH
Men rescue a boy from under the rubble after what activists said was explosive barrels dropped by forces loyal to Syria's President Bashar Al-Assad in Al-Shaar neighbourhood of Aleppo April 6, 2014. REUTERS/Hosam Katan (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

A helicopter loyal to Syrian President Bashar Al-Assad throws what activists say is an explosive barrel, in Al-Shaar area in Aleppo, February 1, 2014. REUTERS/Mohamed Mounzer Masri (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

Smoke rises at a site hit by what activists said were barrel bombs dropped by government forces in the Al-Ansari neighborhood of Aleppo January 31, 2014. REUTERS/Ammar Abdullah (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

A man reacts at a site hit by what activists say was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad, in Aleppo's al-Fardous district April 29, 2015. REUTERS/Hosam Katan 

A family mourns the loss of their relatives to what activists say was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad, in Aleppo's al-Fardous district April 29, 2015. REUTERS/Hosam Katan TPX IMAGES OF THE DAY

Men look through a hole in a wall at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Bab al-Nasr neighborhood in the old city of Aleppo May 25, 2014. Picture taken May 25, 2014. REUTERS/Abdalrhman Ismail (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

Men react near a site hit by what activists say are barrel bombs dropped by government forces on al-Katerji district in Aleppo, January 21, 2014. REUTERS/Hosam Katan (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

A survivor holds her crying baby in a hospital in Aleppo's al-Sakhour district, after what activists said was a barrel bomb was dropped on Aleppo's Haydariye district by forces loyal to Syria's President Bashar al-Assad, March 9, 2014. REUTERS/Hosam Katan (SYRIA - Tags: CIVIL UNREST POLITICS CONFLICT TPX IMAGES OF THE DAY) 

Smoke rises after what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad at Jabal al-Turkman in Latakia province May 11, 2014. Picture taken May 11, 2014. REUTERS/Stringer (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

Workers repair electrical grids as civilians walk past buildings damaged by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's al-Shaar neighborhood February 26, 2015. REUTERS/Hosam Katan (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT ENERGY) 

A man cleans bloodstains off a wall of a shop at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Kfar Zeita village in the central province of Hama May 15, 2014. REUTERS/Ahmad Rif (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

A member of the Civil Defence and a resident pass a fire at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's al-Shaar district April 24, 2014. REUTERS/Hosam Katan (SYRIACIVIL CONFLICT - Tags: CONFLICT CIVIL UNREST TPX IMAGES OF THE DAY) 

Men inspect damage at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's district of al-Sukari March 16, 2014. REUTERS/Ammar Abdullah (SYRIA - Tags: CIVIL UNREST POLITICS TPX IMAGES OF THE DAY CONFLICT) 

Smoke rises after what activists said was a government jet dropping barrel bombs at Qadi Askar district, in Aleppo November 28, 2013. REUTERS/Mahmoud Hebbo (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

A man is seen through a damaged vehicle at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's al-Sakhour district March 12, 2014. REUTERS/Hosam Katan (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

Residents walk amidst the rubble of a site damaged by what activists said was a barrel bomb dropped by forces loyal to Syria's president Bashar Al-Assad in Aleppo's al-Saliheen district, May 1, 2015. REUTERS/Hosam Katan 

A rebel fighter runs through dust towards an area damaged by what activists said were barrel bombs dropped by warplanes loyal to Syria's President Bashar al-Assad in Aleppo's al-Shaar neighborhood November 6, 2014. REUTERS/Rami Zayat (SYRIA - Tags: CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH

An injured man waits for medical attention after, according to activists, two barrel bombs were thrown by forces loyal to Syria's president Bashar Al-Assad in Hullok neighbourhood of Aleppo May 1, 2014. At least 33 people were killed when Syrian government warplanes attacked a busy market in a rebel-controlled district of Aleppo on Thursday, the Syrian Observatory for Human Rights said. REUTERS/Hosam Katan (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

A picture lies amidst debris of collapsed buildings after what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in the Al-Qatrgi neighbourhood of Aleppo, May 1, 2015. REUTERS/Hamid Khatib 

Flames erupt from a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Bab al-Nasr neighborhood in the old city of Aleppo May 25, 2014. Picture taken May 25, 2014. REUTERS/Abdalrhman Ismail (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

Residents inspect a damaged site after what activists said was a barrel bomb dropped by forces loyal to Syria's president Bashar Al-Assad in Maarat Al-Nouman, south of Idlib, May 8, 2015. REUTERS/Khalil Ashawi TPX IMAGES OF THE DAY

A civil defence member is stuck under debris at a site hit by what activists said were two barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in the Al-Shaar neighbourhood of Aleppo July 27, 2014. REUTERS/Stringer (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

A man carrying a child runs out of a burning building at a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad in Aleppo's al-Shaar district July 9, 2014.
REUTERS/Hamid Khatib (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) 

People inspect damage at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's district of al-Sukari March 7, 2014. REUTERS/Hosam Katan (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT) BEST QUALITY AVAILABLE 

Men react at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad, followed by shelling at a bus station in Jisr al-Hajj roundabout in Aleppo May 12, 2015. REUTERS/Hosam Katan 

A Civil defence member looks for survivors at a site hit by what activists said were two barrel bombs dropped by forces loyal to Syria's President Bashar al-Assad in the Al-Shaar neighbourhood of Aleppo July 27, 2014. REUTERS/Hamid Khatib (SYRIA - Tags: POLITICS CIVIL UNREST CONFLICT TPX IMAGES OF THE DAY) 

A boy carries his belongings at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo's al-Fardous district April 2, 2015. REUTERS/Rami Zayat TPX IMAGES OF THE DAY