Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded.
An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations.
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Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens.
Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey.
Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system.
Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011.
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Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said.
Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012.
Bulent Kilic/AFP/Getty Images
Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city.
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A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012.
Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012.
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A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012.
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Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012.
Marco Longari/AFP/Getty Images
Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries.
Rauf Maltas/Anadolu Agency/Reuters/LANDOV
A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012.
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An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War.
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Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012.
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The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city.
Javier Manzano/AFP/Getty Images
A father reacts after the deaths of two of his children in Aleppo on January 3, 2013.
Syrians look for survivors amid the rubble of a building targeted by a missile in the al-Mashhad neighborhood of Aleppo on January 7, 2013.
Achilleas Zavallis/AFP/Getty Images
Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013.
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An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013.
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The UN Security Council passes a resolution September 27, 2013, requiring Syria to eliminate its arsenal of chemical weapons. Al-Assad said he would abide by the resolution.
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Residents run from a fire at a gasoline and oil shop in Aleppo's Bustan Al-Qasr neighborhood on October 20, 2013. Witnesses said the fire was caused by a bullet from a pro-government sniper.
Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013.
An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013.
Mohammed Al-Khateib/AFP/Getty Images
A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014.
A US ship staff member wears personal protective equipment at a naval airbase in Rota, Spain, on April 10, 2014. A former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of Syria's most dangerous chemical agents and sail them out to sea, officials said.
Marcelo del Pozo/Reuters/Landov
A Free Syrian Army fighter fires a rocket-propelled grenade during heavy clashes in Aleppo on April 27, 2014.
A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out.
Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs.
Amr Radwan Al-Homsi/AFP/Getty Images
Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report.
Andrew Harnik/The Washington Times/Landov
Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief.
Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014.
A long-exposure photograph shows a rocket being launched in Aleppo on October 5, 2014.
Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015.
Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015.
A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015.
Bulent Kilic/AFP/Getty Images
A refugee carries mattresses as he re-enters Syria from Turkey on June 22, 2015, after Kurdish People's Protection Units regained control of the area around Tal Abyad, Syria, from ISIS.
Uygar Onder Simsek/AFP/Getty Images
A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015.
Sameer Al-Doumy/AFP/Getty Images
Members of a Syrian opposition group attack the headquarters of al-Assad regime forces in the Aleppo villages of Nubul and al-Zahraa on February 12, 2016.
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This still image, taken from a video posted by the Aleppo Media Center, shows a young boy in an ambulance after an airstrike in Aleppo, Syria, on August 17, 2016. It took nearly an hour to dig the boy, identified as Omran Daqneesh, out from the rubble, an activist told CNN. The airstrike destroyed his home, where he lived with his parents and two siblings. Director of the Aleppo Media Center Yousef Saddiq said Omran's 10-year-old brother, Ali, died from his injuries.
Mahmud Rslan/Anadolu Agency/Getty Images
Syrian men carrying babies make their way through the rubble of destroyed buildings following a reported air strike on the rebel-held Salihin neighborhood of the northern city of Aleppo, on September 11, 2016.
Ameer Alhalbi/AFP/Getty Images
Smoke rises after an airstrike in Aleppo on October 4, 2016.
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Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces.
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This photo, provided by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack.
Idlib Media Center
Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria.
Kena Betancur/AFP/Getty Images
Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017.