This picture taken the United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) shows residents of the besieged Palestinian camp of Yarmouk, queuing to receive food supplies, in Damascus, Syria, on January 31, 2014. A United Nations official is calling on warring sides in Syria to allow aid workers to resume distribution of food and medicine in a Palestinian district of Damascus. The call comes as U.N. Secretary General Ban Ki-Moon urged Syrian government to authorize more humanitarian staff to work inside the country, devastated by its 3-year-old conflict. #

A Free Syrian Army fighter runs to take cover after launching a mortar towards forces loyal to Syria's President Bashar al-Assad in the Jabal al-Akrad area of Syria's northwestern Latakia province, on February 25, 2014. #

Damaged buildings line a street in the besieged area of Homs on January 27, 2014. The United States demanded that Syria allow aid into the "starving" city of Homs, as talks aimed at ending three years of civil war took place. #

Residents of the besieged Yarmouk Palestinian refugee camp wait to leave the camp, on the southern edge of the Syrian capital Damascus, Syria, on February 4, 2014. In one besieged neighborhood after another, weary rebels have turned over their weapons to the Syrian government in exchange for an easing of suffocating blockades that have prevented food, medicine and other staples from reaching civilians trapped inside. #

A picture taken on March 15, 2014 shows part of the seven-square-kilometer (2.8-square-mile) Zaatari refugee camp in northern Jordan, near the border with Syria, which provides shelter to around 100,000 Syrian refugees. Syrian refugees in the sprawling desert camp fear that President Bashar al-Assad's likely re-election this year will leave their dream of a return home as distant as ever. Jordan is home to more than 500,000 of Syria's refugees. #

Rebel fighters hold a position as smoke billows in the background during reported battles against pro-government forces on February 23, 2014, in the Sheikh Najjar district on the outskirts of the northern Syrian city of Aleppo. #

A pro-government fighter flashes the "V-sign" for victory next to comrades in the Syrian town of Yabrud, after they seized full control of a rebel-controlled area in the strategic Qalamun region near the Lebanese border, on March 16, 2014. The town was once home to some 30,000 people, including a Christian minority, and had been a rebel bastion since early in the Syrian uprising that began in March 2011. #

Wreckage of a Syrian army helicopter after members of the al-Qaeda-linked group Islamic State of Iraq and the Levant (ISIL) allegedly destroyed it, in the northern Syrian city of Aleppo, to prevent other opposition groups from taking it. Photographed on March 1, 2014. #

Free Syrian Army members lower the body of their fellow fighter, who the FSA say was killed during an explosion inside a tunnel caused by forces loyal to President Bashar al-Assad, during his funeral in Deir al-Zor, on February 20, 2014. #

Free Syrian Army fighters Mohamad-Noor (left), 14, and Hadi, 15, show a picture of the body of their 15-year-old friend Khaled, who was killed by sniper fire, at the frontline of Khalidiya neighborhood in Aleppo on March 13, 2014. Noor and Hadi joined the Free Syrian Army 6 months ago along with Khaled. Noor's father died from sniper fire too, and Hadi's father fights with the Free Syrian Army. #

Khattab al-Halabi, a former tattoo artist who is now a Free Syrian Army fighter, poses with his weapon in at the Karm al-Jabal frontline in Aleppo on February 23, 2014. #

Two Syrian men who fled from Yabroud, the last rebel stronghold in Syria's mountainous Qalamoun region, set up a tent in Wadi Hmaied between the Lebanese-Syrian border and the town of Arssal, in eastern Lebanon, on February 12, 2014. In Lebanon, preparations were underway to receive more Syrians fleeing the area. #

Eyad, a 12 year-old boy who lost his arm during shelling by forces loyal to Syria's President Bashar al-Assad, tries on a prosthetic arm at Duma Charity Foundation for Prosthesis in the Duma neighborhood of Damascus on February 25, 2014. The center produces prostheses from different materials, including remnants of weapons, plastic mannequins and water barrels, and offers the devices to people who have lost their limbs during the war. #

Rebel fighters fire a heavy machine gun at the government forces Al-Samman checkpoint on a road leading to Idlib, near the Syrian city of Hama, on February 17, 2014. #

In this photo provided by Aleppo Media Center (AMC), an anti-Bashar Assad activist group, which has been authenticated based on its contents and other AP reporting, a Syrian government forces helicopter drops what activists said are two barrel bombs over an area of Aleppo on February 2, 2014. For nearly two months, the Syrian government has conducted an intense air campaign on opposition-held districts of the northern city of Aleppo. #

This photo provided by the anti-government activist group Aleppo Media Center (AMC), which has been authenticated based on its contents and other AP reporting, shows a Syrian man, right, running for cover from a Syrian government forces airstrike attack in Aleppo on February 27, 2014. #

This picture provided by the anti-government activist group Coordination Committee In Kfar Takharim, which has been authenticated based on its contents and other AP reporting, shows a Syrian man running as flames rise from buildings which were attacked by a Syrian government forces airstrike in the town of Kfar Takharim in the northwest province of Idlib, on March 1, 2014. #

Men carrying children run from a burning building following a barrel bomb attack reportedly dropped by government forces in the northern Syrian city of Aleppo on February 8, 2014. #

A Syrian man breathes through an oxygen mask while awaiting rescue, trapped under rubble following an air strike by government forces on the Sahour neighborhood of Aleppo on March 6, 2014. #

Men hold up a baby saved from under rubble, who survived what activists say was an airstrike by forces loyal to Syrian President Bashar al-Assad in the Duma neighborhood of Damascus on January 7, 2014. The child, 27-day-old Rateb Malis, was later reunited with his father and sisters. #

A Syrian national flag flutters atop of a damaged building in a government-controlled district, as seen from a rebel-held area, in Aleppo's Karm al-Jabal district on March 17, 2014. #

Flames rise from an explosion after opposition fighters hit a government forces tank in Khan Sheikhun, in the south of the Syria's northwestern Idlib province, on February 26, 2014. #

People inspect damage at a site hit by what activists said were barrel bombs dropped by forces loyal to President Bashar al-Assad in Aleppo's district of al-Sukari on March 7, 2014. #

Debris covers a street and flames rise from a building following a reported air strike by Syrian government forces on March 7, 2014 during the Friday prayer in the Sukkari neighborhood of Aleppo. #