An online video has showed more than a dozen bloodied corpses, some of them piled on top of each other and in military uniforms, dumped beside a road in northern Syria in what the government called a mass killing by rebel forces.
The circumstances of the deaths were not immediately clear, with the state-run news agency saying at least 25 men were killed.
In the video, the narrator said the victims were members of the "shabiha," or pro-regime gunmen. If confirmed, the video is yet another sign of the brutality of the Syrian conflict, which began in March 2011.
As the fighting grinds on, Syria is descending into a civil war where gunmen prowl the streets and gruesome massacres are growing increasingly common.
The government has used heavy weapons and unleashed snipers and loyalist fighters, but rebels, too, have been accused of bloody attacks.
Civilians have been caught in the crossfire; activists estimate that more than 14,000 people have been killed since the start of the uprising against President Bashar Assad's regime.
In a desperate bid to end the violence after an earlier peace plan failed to do so, UN envoy Kofi Annan said that Iran - one of Syria's most loyal allies - should be part of the solution to the conflict.
Syria's state-run news agency, Sana, said the dead found in the rebel-held area of Daret Azzeh near Aleppo were killed and their bodies mutilated by terrorist groups. The government refers to rebels as terrorists.
The amateur video showing the corpses appeared to back up the allegation of a mass killing. "The terrorist groups in Daret Azzeh committed a brutal massacre against the citizens, whom they had kidnapped earlier in the day," Sana said.
The report said at least 25 people were killed, but others were missing.