A Syrian man reacts while standing on the rubble of his house while others look for survivors and bodies in the Tariq al-Bab district of the northern city of Aleppo on February 23, 2013. Pablo Tosco/AFP/Getty Images
A Free Syrian Army fighter aims his weapon during clashes with government forces in Aleppo on Tuesday, January 15, 2013. Andoni Lubaki/AP
A member of the Syrian pro-government forces stands amid heavily damaged buildings in Aleppo's 1070 district on November 8, 2016, after troops seized it from rebel fighters. GEORGES OURFALIAN/AFP/Getty Images
A wounded Syrian boy cries after bombs fell on the opposition-controlled Firdevs neighborhood in Aleppo on October 11, 2016. Jawad al Rifali/Anadolu Agency/Getty Images
Smoke rises after a bomb explodes in a residential area in the Darat Izza neighborhood of Aleppo on October 4, 2016. Mahmuh Faysal/Anadolu Agency/Getty Images
Search and rescue team members carry an injured man after Syrian regime airstrikes targeted the Meshed neighborhood of Aleppo on July 21, 2016. Beha el Halebi/Anadolu Agency/Getty Images
A rebel fighter aims his weapon toward Syrian government forces' positions at the Menagh military airport near Aleppo on March 13, 2013. JM LOPEZ/AFP/Getty Images
The body of a Syrian army soldier lies on the ground after heavy clashes with government forces at a military academy besieged by the rebels in Tal Sheer village, north of Aleppo, on December 16, 2012. Manu Brabo/AP
Debris covers a street and flames rise from a building after a reported airstrike by Syrian government forces on March 7, 2014, during the Friday prayer in the Sukkari neighborhood of Aleppo. Baraa al-halabi/AFP/Getty Images
Wounded 5-year-old Omran Daqneesh sits alone in the back of an ambulance after he was injured during a Russian or Assad regime forces airstrike targeting the Qaterji neighborhood of Aleppo on August 17, 2016. Mahmud Rslan/Anadolu Agency/Getty Images A Syrian opposition tank fires a rocket toward an Assad forces' building during clashes near the Air Intelligence building of Jamiat al-Zahra, Aleppo, on April 13, 2015. Mahmoud Faisal/Anadolu Agency/Getty Images
On September 7, 2012, Free Syrian Army fighters run after attacking a Syrian army tank during fighting in the Izaa district of Aleppo. Manu Brabo/AP
Search and rescue team members inspect collapsed buildings after Assad regime forces attacked residential areas in the Karm al-Beik region of Aleppo on July 9, 2015. Beha El Halebi/Anadolu Agency/Getty Images
Syrian civil defense volunteers and rescuers remove a baby from under the rubble of a destroyed building after a reported airstrike on the rebel-held neighborhood of al-Kalasa in Aleppo on April 28, 2016. AMEER ALHALBI/AFP/Getty Images
A Free Syrian Army fighter takes cover during clashes with Syrian army soldiers in the Salaheddine neighborhood of central Aleppo on August 7, 2012. Goran Tomasevic/REUTERS/Newscom
Syrian pro-government forces walk in the damaged ancient Umayyad Mosque in the old city of Aleppo on December 13, 2016, after they captured the area. GEORGE OURFALIAN/AFP/Getty Images
A man in front of a field hospital mourns the death of his relatives on August 21, 2012, in Aleppo. Edouard Elias/Getty Images
Rubble is seen in the Salaheddine neighborhood on March 24, 2013, in Aleppo. Antonio Bolfo/Getty Images
Syrian government forces walk in the strategic area of the Bazo hilltop, on the southern outskirts of Aleppo, as they advance to seize the rebel-held eastern part of the city on October 25, 2016. GEORGES OURFALIAN/AFP/Getty Images
An aerial view shows a convoy of buses and ambulances waiting at a crossing point in the Amiriyah district of Aleppo on December 15, 2016, to evacuate civilians trying to flee from areas under siege by Iran-led Shiite militias and Assad regime forces. Hasan Katan/Anadolu Agency/Getty Images