A rebel fighter of the Al-Furqan brigade covers his ears as a fellow fighter fires a vehicle's weapon during what the rebels said is an offensive to take control of the al-Mastouma army base which is controlled by forces loyal to Syria's President Bashar al-Assad near Idlib city May 17, 2015. Picture taken May 17, 2015. REUTERS/Khalil Ashawi TPX IMAGES OF THE DAY

Bloodstains are seen at a school where a Syrian teacher was killed by a mortar shell in Damascus May 20, 2015. A Syrian teacher was killed and 23 students were wounded after a mortar shell hit their school on Wednesday in the heart of Damascus, Syrian state media said. REUTERS/Omar Sanadiki 

Residents, with their belongings, return to their villages on a pickup truck after Kurdish People's Protection Units (YPG) fighters said that they retook control of the area from Islamic State in the southern countryside of Ras al-Ain May 13, 2015. Picture taken May 13, 2015. REUTERS/Rodi Said TPX IMAGES OF THE DAY

Tajammu Al-Ezza brigade fighters react while firing a Grad shell towards forces of Syria's President Bashar Al-Assad stationed in Salhab village, Hama countryside, May 7, 2015. REUTERS/Mohamad Bayoush 

Smoke rises after a shell fell on a building that forces loyal to Syria's president Bashar Al-Assad are located in, after being fired from rebel fighters in the Seif El Dawla neighbourhood in Aleppo March 28, 2015. REUTERS/Rami Zayat 

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY

A boy, injured from what activists said was shelling by forces loyal to Syria's President Bashar al-Assad in Ain Larouz village in the Jabal al-Zawiya region, reacts at a field hospital in Idlib province, May 15, 2015. REUTERS/Abed Kontar TPX IMAGES OF THE DAY

A rebel fighter uses a military compass in preparation to launch a home made shell towards forces loyal to Syria's president Bashar Al-Assad in the Seif El Dawla neighbourhood in Aleppo March 28, 2015. REUTERS/Rami Zayat 

Shamiyah Front fighters walk up the stairs of a damaged building carrying a locally made shell before firing it towards forces loyal to Syria's President Bashar Al-Assad in the old city of Aleppo April 10, 2015. Picture taken April 10, 2015. REUTERS/Abdalrhman Ismail

A rebel fighter from 'Jaysh al-Sunna' with blood on his forehead sits in a trench during what they said was an offensive to take control of al-Mastouma military camp which is controlled by forces loyal to Syria's President Bashar al-Assad beside Idlib city May 12, 2015. Picture taken May 12, 2015. REUTERS/Khalil Ashawi 

A rebel fighter of 'Al-Sultan Murad' brigade fires an anti-tank missile towards forces loyal to Syria's President Bashar al-Assad near Aleppo's international airport May 9, 2015. REUTERS/Rami Zayat 

A rebel fighter looks under a roller shutter in Mastouma village, after they said they had advanced on the area, in Idlib city May 19, 2015. Heavy clashes between Syria's army and insurgents erupted around a military base south of Idlib city on Monday as part of a wider battle for control of the northwestern province, Syrian state television and a group monitoring the war said. REUTERS/Ammar Abdullah 

A rebel fighter rides a motorbike along a road in Qarmeed camp, as smoke rises in the background after Islamist rebel fighters took control of the area from forces of Syria's President Bashar al-Assad, April 27, 2015. A coalition of Islamist rebels seized the army base in northwestern Syria at dawn on Monday after a suicide bomber from al Qaeda's Nusra Front drove a truck packed with explosives into the compound and blew it up. The capture, reported by a rebel commander and social media videos showing militants inside the base, brought the coalition closer to seizing most of Idlib province and moving toward Latakia, the ancestral home of Assad. REUTERS/Abdalghne Karoof TPX IMAGES OF THE DAY

A Civil Defence member carries a damaged canister in Ibleen village from what activists said was a chlorine gas attack, on Kansafra, Ibleen and Josef villages, Idlib countryside May 3, 2015. REUTERS/Abed Kontar 

A Jaysh al-Islam (Army of Islam) rebel fighter fires his weapon towards forces loyal to Syria's President Bashar Al-Assad at the Tal-Kurdi frontline in the Eastern Ghouta of Damascus, May 10, 2015. Jaysh al-Islam (Army of Islam) fighters said that this was an operation to take control of the highway connecting Damascus to Homs, and that they have made advances in Tal-Kurdi. REUTERS/Amer Almohibany

Debris and smoke rise during what rebel fighters said was an operation in which they blew up a tunnel targeting a post controlled by forces loyal to Syria's President Bashar al-Assad, in Ariha town in Idlib province May 12, 2015. Rebel fighters announced on Tuesday the start of an offensive to take control of Ariha town from government forces. REUTERS/Ammar Abdullah 

The Syrian national flag flutters from a damaged building as seen from a rebel-held area in Aleppo's Al-Ezaa neighbourhood May 20, 2015. REUTERS/Hosam Katan 

A Jaysh al-Islam (Army of Islam) rebel fighter fires his weapon towards forces loyal to Syria's President Bashar Al-Assad as another fighter covers his ear at the Tal-Kurdi frontline in the Eastern Ghouta of Damascus May 10, 2015. Jaysh al-Islam (Army of Islam) fighters said that this was an operation to take control of the highway connecting Damascus to Homs, and that they have made advances in Tal-Kurdi. REUTERS/Amer Almohibany TPX IMAGES OF THE DAY

A warplane that belongs to forces loyal to Syria's president Bashar Al-Assad flies over the northwestern city of Idlib March 26, 2015. An alliance of Syrian Islamist rebels including al Qaeda's official wing said they had launched an offensive on Tuesday to take the northwestern city of Idlib, which is close to the main strategic highway linking Damascus to Aleppo. Picture taken March 26, 2015. REUTERS/Khalil Ashawi 

Kurdish People's Protection Units (YPG) fighters walk amid debris near damaged silos of Aliya village after saying that they retook control of the area from Islamic State in the southern countryside of Ras al-Ain May 13, 2015. Picture taken May 13, 2015. REUTERS/Rodi Said 

Rebel fighters pose for a picture with their weapons in Idlib city, after they took control of the area March 28, 2015. REUTERS/Khalil Ashawi

A Free Syrian Army fighter covers his ears as his fellow fighter fires a weapon during clashes with forces loyal to Syria's President Bashar al-Assad in the old city of Aleppo April 30, 2015. Fighters from 'the First Regiment' and 'al-Safwa Brigades', part of the Free Syrian Army, said on Friday that they took control of Bashir Basha mosque after heavy clashes with government forces in the old city of Aleppo. REUTERS/Hosam Katan TPX IMAGES OF THE DAY

A Kurdish People's Protection Units (YPG) fighter stands atop of a burnt tank that belonged to Islamic State fighters in the Aliya village after saying that they retook control of the area from IS in the southern countryside of Ras al-Ain May 13, 2015. Picture taken May 13, 2015. REUTERS/Rodi Said TPX IMAGES OF THE DAY

Lebanon's Hezbollah fighters rest on the back of a pick-up truck with an installed weapon in Khashaat, in the Qalamoun region after they advanced in the area May 15, 2015. Lebanon's Hezbollah and Syria's army made big advances against insurgents in mountains north of Damascus on Wednesday, Hezbollah and Syrian state media said, shoring up President Bashar al-Assad's grip on the border zone. REUTERS/Mohamed Azakir TPX IMAGES OF THE DAY

Smoke rises after what activists said were clashes between Al-Rahman corps and forces of Syria's President Bashar al-Assad in Al-Hajez garages, near the Damascus countryside governorate building at the southern entrance to Erbeen in the eastern Damascus suburb of Ghouta May 16, 2015. The Al-Hajez garages and the area around it are under the control of forces of Syria's President Bashar Al-Assad. REUTERS/Amer Almohibany TPX IMAGES OF THE DAY

A view is seen through the scope of a weapon belonging to an Ahrar al-Sham Islamic Movement rebel fighter in the northwestern city of Idlib March 26, 2015. An alliance of Syrian Islamist rebels including al Qaeda's official wing said they had launched an offensive on Tuesday to take the northwestern city of Idlib, which is close to the main strategic highway linking Damascus to Aleppo. Picture taken March 26, 2015. REUTERS/Khalil Ashawi 

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

A rebel fighter fires his weapon at the frontline during what the rebels called a battle to unite rebel factions against forces loyal to Syria's President Bashar al-Assad in Jobar, a suburb of Damascus April 21, 2015. Picture taken April 21, 2015. REUTERS/Amer Almohibany TPX IMAGES OF THE DAY

Rebel fighters place shoes on a damaged sculpture of late Syrian president Hafez al-Assad, father of Syrian President Bashar al-Assad, in Idlib city, after they took control of the area March 28, 2015. REUTERS/Khalil Ashawi