Armed Syrian Turkmen villagers are seen near the northern Syrian village of Yamadi, near the Turkish-Syrian border, Syria, November 24, 2015. Turkey shot down a Russian warplane near the Syrian border on Tuesday saying it had repeatedly violated its air space, one of the most serious publicly acknowledged clashes between a NATO member country and Russia for half a century. (Photo by Reuters/Stringer)

Free Syrian Army fighters stand near an ambulance on the eastern edge of the northern Syrian town of al-Bab, Syria, February 27, 2017. (Photo by Khalil Ashawi/Reuters)

A Syrian boy receives treatment at a hospital in the regime-held part of Aleppo on October 13, 2016. Syrian state television said four children were killed by rebel rocket fire on a school in a western regime-held neighbourhood. (Photo by George Ourfalian/AFP Photo)

Turkish Kurds watch the Syrian town of Kobani from near the Mursitpinar border crossing, on the Turkish-Syrian border in the southeastern town of Suruc, October 19, 2014. (Photo by Kai Pfaffenbach/Reuters)

Internally displaced Syrians who fled Raqqa tie ropes for their tents in a camp in Manbij, Syria on April 6, 2017. (Photo by Rodi Said/Reuters)

Aisha, 15, (L) (who asked to withhold her last name), a Syrian refugee from Raqqa, waits with a fellow refugee while harvesting cannabis in the Bekaa valley, Lebanon October 19, 2015. Syrian refugees work to harvest and process spiky-leafed cannabis plants in neighbouring Lebanon’s Bekaa Valley. Often farmers of cotton and wheat back home in Raqqa province – now the de facto capital of Islamic State – the conflict in Syria drove them to seek safety in a region where Syrian migrant workers used to spend a few months a year before returning home. (Photo by Alia Haju/Reuters)

In this Tuesday, July 29, 2014, photo, Syrian refugee Samah, 5, poses for a picture at Zaatari refugee camp, near the Syrian border, in Mafraq, Jordan. More than 2.8 million Syrian children inside and outside the country – nearly half the school-aged population – cannot get an education because of the devastation from the civil war, according to the U.N. children's agency, UNICEF. That number is likely higher, as UNICEF can't count the children whose parents didn't register with the United Nations refugee agency. (Photo by Muhammed Muheisen/AP Photo)

Mohammad, a 13-year-old fighter from the “Free Syrian Army”, plays with a cat in Aleppo's Bustan al-Basha district, Syria October 28, 2013. Mohammad joined the Free Syrian Army after his father died during clashes with the Syrian regime. The gun he is using was his father's. (Photo by Molhem Barakat/Reuters)