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.
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.
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.
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.
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.
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.
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.
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.
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.
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 <a href="http://www.cnn.com/2016/08/17/world/syria-little-boy-airstrike-victim/" target="_blank">Omran Daqneesh</a> 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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