At least 23 people were "summarily executed" by regime forces in two Damascus neighbourhoods on Sunday, the head of the Syrian Observatory for Human Rights watchdog told AFP on Monday.
"Sixteen people, most of them younger than 30, were summarily executed by shooting on Sunday in Mazzeh," Observatory chief Rami Abdel Rahman said, referring to an upscale district in western Damascus.
The bodies of some of those killed showed signs of torture.
Seven others were executed in a similar fashion in Barzeh in the city's northeast, he said.
It was unclear whether the executions were of civilians or rebel fighters.
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The heads of two of the victims had been crushed by vehicles, and one was shot through the eye, the Britain-based Abdel Rahman said.
Three of the dead were found with their hands tied, and the bodies of some had been pierced by bayonets, he added.
Overnight, Syrian state television reported an assault on Mazzeh, calling the operation "targeted and quick."
Fifteen unidentified bodies were also found Monday in Maadamiyat al-Sham village near Damascus, the Observatory said. Some of them had their hands tied, while others showed signs of having been stabbed.
For its part rights watchdog Amnesty International urged regime and rebel forces to "respect the rules of international humanitarian law which aim at sparing civilians and others not directly participating in the fighting and minimising human suffering."
"Superiors and commanders have a duty to prevent and, where necessary, to suppress war crimes by those under their command or who they otherwise control; and they may be held criminally responsible if they fail to do so."