Rebels from the Free Syrian Army walk on a street in Damascus in this picture provided by Shaam News Network and taken March 23. The Syrian capital came under mortar fire on Sunday and Monday. Ward Al-Keswani/Shaam News Network/Reuters /Landov hide caption

itoggle caption Ward Al-Keswani/Shaam News Network/Reuters /Landov

Um Ahmed, her husband, Abu Ahmed, and their two children sit inside a United Nations refugee registration center in Tripoli, Lebanon. They fled the northern Syrian city of Aleppo a month ago. "There was a lot of shelling," said Um Ahmed. "I wasn't thinking. I was just thinking of my children." Nicole Beemsterboer/NPR hide caption

itoggle caption Nicole Beemsterboer/NPR

Demonstrators wave Syrian opposition flags during a protest in Istanbul against the government of Syria's President Bashar Assad, on March 15. Osman Orsal/Reuters /Landov hide caption

itoggle caption Osman Orsal/Reuters /Landov

Syrians carry a large revolution flag and chant slogans during a protest in Aleppo, Syria, where young people and children sang songs against President Bashar Assad and the Syrian regime, Dec. 21, 2012. Virginie Nguyen Hoang/AP hide caption

itoggle caption Virginie Nguyen Hoang/AP

A rebel fighter raises his weapon after firing a missile Sunday toward Syrian government troops in the northern city of Aleppo. Syria's largest city has been the scene of heavy fighting for the past three months. Both sides control part of the city, and the fight has been a stalemate recently. Narciso Contreras/AP hide caption

itoggle caption Narciso Contreras/AP

An image from amateur video released by Ugarit News purports to show Aleppo under fire. The AP cannot independently confirm the authenticity of the image. AP hide caption

itoggle caption AP