A man reacts over dead bodies prior to burial in a graveyard after shelling in the rebel held besieged town of Douma, eastern Ghouta in Damascus, Syria, October 24, 2016. (Photo by Bassam Khabieh/Reuters)

A Syrian rebel walks past Sham 2, a homemade armored vehicle, in Bishqatin, Syria, on December 8, 2012. From a distance it looks rather like a big rusty metal box but closer inspection reveals a homemade armored vehicle waiting to be deployed. Sham II, named after ancient Syria, is built from the chassis of a car and touted by rebels as “100 percent made in Syria”. (Photo by Herve Bar/AFP Photo)

A “Free Syrian Army” fighter in the Bustan Pasha neighborhood of Aleppo, Syria, August 21, 2013. (Photo by James Lawler Duggan)

Internally displaced Syrian children who fled Raqqa city stand near their tent in Ras al-Ain province, Syria January 22, 2017. (Photo by Rodi Said/Reuters)

A young Free Syrian Army fighter holds a rifle in Haresta neighbourhood of Damascus January 16, 2013. (Photo by Goran Tomasevic/Reuters)

A Russian air force technician reports to a pilot that a bomber is ready for a combat mission at Hemeimeem air base in Syria Wednesday January 20, 2016. Russian warplanes have flown over 5,700 combat missions since Moscow launched its air campaign in Syria on September 30, 2015. (Photo by Vladimir Isachenkov/AP Photo)

This file photo provided on Sunday February 15, 2015 by the Syrian anti-government activist group Aleppo Media Center (AMC), which has been authenticated based on its contents and other AP reporting, shows Syrian rebels firing locally made shells against the Syrian government forces, in Aleppo, Syria. (Photo by AP Photo/Aleppo Media Center)

A Russian Sukhoi Su-25 fighter jet takes off during the withdrawal of Russian troops from Syria at Hmeymim airbase, Syria, March 16, 2016. (Photo by Vadim Grishankin/Reuters/Russian Ministry of Defence)