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.
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.
On Thursday evening, activists posted on the internet two videos showing the bodies of the 13 men who they said had been killed in al-Buwaida al-Sharqiya, a village between Qusair and the city of Homs, earlier that day.
One video showed a group of bodies sprawled on the ground, with hideous injuries consistent with their having been shot dead at close range in the head or stomach, reports the BBC's Jim Muir in Beirut.
Another video showed the bodies laid out on the floor of a building, with relatives grieving over them, our correspondent adds.
Activists said the murdered men were workers from a fertiliser factory whose bus was intercepted by shabiha members. They first of all robbed the workers, then took them off the bus and executed them, the activists added.