A fighter belonging to Jaish al-Islam (Islam Army) runs to take cover near the frontline on March 14, 2016 in the neighbourhood of Jobar, on the eastern outskirts of the capital Damascus. (Photo by Amer Almohibany/AFP Photo)

A Russian ground staff member signals to the pilot of a Sukhoi Su-25 fighter jet at the Hmeymim air base near Latakia, Syria, in this handout photograph released by Russia's Defence Ministry October 22, 2015. (Photo by Reuters/Ministry of Defence of the Russian Federation)

A Syrian Democratic Forces (SDF) female fighter adjusts a mortar in northern Deir al-Zor province ahead of an offensive against Islamic State militants, Syria February 21, 2017. (Photo by Rodi Said/Reuters)

A little girl holding her newborn sister waits for their turn in a queue during a vaccination campaign under the supervision of the Syrian Arab Red Crescent (SARC) humanitarian nonprofit organization, in the besieged Syrian town of Douma, Syria, 23 May 2017. According to a statement from SARC, in the city of Douma some 12,809 children under the age of five were immunized in a vaccination campaign to prevent measles and poliomyelitis. The campaign was the first of its kind with the participation of 100 volunteers. SARC has been delivering vaccines to 17 health centers approved for vaccination campaigns in towns and villages in the Eastern Ghouta area. (Photo by Mohammed Badra/EPA)

Men pull as a boy stands on a soap cutter in a soap factory in Idlib province, Syria January 30, 2016. (Photo by Ammar Abdullah/Reuters)

A young man with an amputated leg poses for a photograph standing by his broken artificial limb in the rebel-controlled area of Maaret al-Numan town in Idlib province, Syria March 20, 2016. Two university students forced to interrupt their studies have learnt to make and fit hundreds of new limbs in the past four years in opposition-held areas of Syria. A mobile clinic operating from a truck has gone some way to improve access to treatment. While most patients are between 15 and 45, the clinic also helps children and the elderly with replacement limbs. (Photo by Khalil Ashawi/Reuters)

Girls sit inside an empty classroom as they pose for a photograph during a celebration marking the end of the school year in the town of Douma, eastern Ghouta in Damascus, Syria May 21, 2016. (Photo by Bassam Khabieh/Reuters)

A man stands near bodies of his young relatives after an airstrike in the rebel held Douma neighbourhood of Damascus, Syria August 22, 2016. (Photo by Bassam Khabieh/Reuters)