Thirteen factory workers were forced off a bus and executed by shabiha members in a village near Qusair, in the west of the country, they said.
Correspondents say the video shows a group of bodies with hideous injuries.
The UN Human Rights Council has meanwhile begun an emergency session to discuss the violence in Syria.
It is expected to blame pro-government forces for last week's massacre in Houla, in which more than 100 people died, including 49 children.
On Thursday, a Syrian government investigation into the killings blamed armed rebel groups seeking to trigger foreign military intervention.
The US permanent representative to the UN, Susan Rice, dismissed the finding as a "blatant lie", for which there was no factual evidence.