Eighteen unidentified bodies were found in the Damascus area on Saturday, the Syrian Observatory for Human Rights said, adding that summary executions by both sides in the conflict were on the increase.
Most of the bodies had their hands tied and some showed signs of torture, the Britain-based watchdog said.
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In second city Aleppo, meanwhile, state media said that a "terrorist" group had killed five members of a family in the central Marjeh district.
The official SANA news agency identified the dead as Ali Merhi, his brother Khaled, 42, and "three children under the age of 17".
On August 15, a UN Commission of Inquiry said it had found evidence that Syrian government forces and their militia allies had committed crimes against humanity during the conflict now in its 18th month.
It found that rebel fighters were also guilty of crimes, albeit on a smaller scale.