Dozens of bodies were found on Saturday in a besieged town near Damascus where Syrian troops have been waging fierce attacks to try to crush rebel holdouts in the capital, a monitoring group said.
At least 15 civilians, including two women and three children, were also killed on Saturday in the town of Daraya, which lies just southwest of Damascus, the Syrian Observatory for Human Rights said.
According to figures from the Britain-based watchdog, more than 120 people, including civilians and rebels, have been killed in the army onslaught on the town this week.
It said the 15 were killed either by gunfire or were summarily executed on Saturday, in addition to between 40 and 50 bodies that it said were discovered near a mosque.
"Security forces have launched a campaign of arrests and residents are anxious and afraid that there will be another civilian massacre" in Daraya, said the Local Coordination Committees, a network of activists on the ground.
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Syrian state television said that Daraya was being "purified of terrorist remnants."
The Observatory reported that Syrian troops had launched major arrest sweeps in Daraya and were clashing with rebels.
The army claimed to have retaken most of Damascus in late July, after some two weeks of intense fighting across the capital's southern belt. Most rebel Free Syrian Army fighters were forced out into the nearby countryside, but have since resumed hit-and-run operations, activists say.
August is already the deadliest single month of the Syria conflict with more than 4,000 people killed, according to the Observatory, which reported at least 63 people killed nationwide on Saturday, the majority of them civilians.
More than 24,500 have been killed since the uprising erupted in March last year, the watchdog said. The United Nations puts the death toll at more than 17,000.
The Observatory relies on a network of local activists, doctors and other sources, but its reports cannot be independently verified because of tight curbs on media in Syria.