Syrian regime opponents said that a video published online on Saturday shows the dead bodies of people killed during a military offensive on the town of Daraya, close to the capital Damascus.
The opposition said as many as 270 people were killed execution-style as the army retook the area from rebel fighters.
Due to restrictions on non-state media, it is impossible to independently verify reports from Syria.
The British-based Syrian Observatory for Human Rights said it had received reports of several dozen people being killed across the country on Saturday.
Attacks in the city of Aleppo have also continued unabated.
Rebels are said to be in control of the northern part of the commercial capital, but they are coming under increasing assault from Syrian military combat jets and helicopters.