Reports suggest Syrian government airstrikes northeast of the capital have killed at least 80 people.
Images taken from social media purport to show the aftermath of an attack on a market in Douma, some 15 kilometres from Damascus.
Rescue workers said at least 200 people were wounded in the assault.
The British-based Syrian Observatory for Human Rights said warplanes fired around ten rockets and bombed again after rescue workers had arrived on the scene.
A source in the Syrian military claimed airstrikes in Douma and neighbouring Harasta targeted the headquarters of the Islam Army rebel group.