A Syrian government air strike northeast of Damascus killed at least 80 people in a marketplace on Sunday, rescue workers in the rebel-held area and the Syrian Observatory for Human Rights said.
At least 200 more people were wounded in the attack on Douma, about 15 km (10 miles) northeast of Damascus, according to the British-based Observatory and the local arm of Syrian Civil Defense, a rescue service operating in rebel areas.
The Observatory said at least 10 rockets were fired during the air strike and accused the warplanes of bombing again after rescue workers had arrived at the scene.
"This is a documented massacre," said Rami Abdulrahman, who runs the Observatory.
A Syrian military source said air force strikes on Douma and nearby Harasta targeted the headquarters of the rebel group Islam Army. The insurgent group attacked government positions in Harasta on Saturday.
A video released by Islam Army showed its leader Zahran Alloush taking part in the attack. The group is one of the most powerful insurgent factions operating near Damascus, the seat of President Bashar al-Assad's power more than four years into the civil war that has killed a quarter of a million people.
Another video bearing the symbol of the Syrian Civil Defense rescue service appeared to show dozens of bodies lined up on the ground. A photo showed civil defense workers wrapping bodies in white shrouds.
Another video showed rescuers carrying bodies in a street strewn with vegetables and debris.
Syrian Civil Defense published the names of 60 people so far identified as killed in the attack. "There are tens waiting to be identified," it said on its Facebook page.
An official for the rescue service told Reuters the videos it published were shot on Sunday and a person heard on camera giving the date as August 15 had done so by mistake.
To the west of Damascus, battles were reported on Sunday between the Syrian army, backed by its Lebanese ally Hezbollah, and insurgents in the town of Zabadani near the Lebanese border, the Observatory said.
A ceasefire between the warring sides in Zabadani and two villages in the northwest collapsed on Saturday when talks aimed at securing a longer cessation of hostilities failed.