A video distributed on Thursday by the Syrian Observatory for Human Rights shows jihadists in the east of the country executing supporters of the regime of President Bashar al-Assad.
The Britain-based watchdog did not specify whether the men killed were combatants, nor did it say when the footage was shot.
The men killed by Al-Nusra Front, a jihadist organisation operating in Syria that the United States has classed as "terrorist" "are supporters of the Assad regime, but we cannot know whether they were regime troops", watchdog director Rami Abdel Rahman told AFP.
The online video shows a masked man reading out a statement. At least nine blindfolded men kneel in front of him, facing the camera.
Behind the men are two black flags.
"Al-Nusra Front's sharia (Islamic law) court has in the eastern area of Deir Ezzor (near the Iraqi border) sentenced these apostate soldiers to death, because of their massacres against our people in Syria," the unidentified jihadist fighter says in the video.
The man then takes a pistol and shoots the prisoners, one by one.
Jihadist websites also posted the video, and said the footage was filmed in 2012.
In an act that enraged other Islamists fighting Assad's regime in Syria, Al-Nusra Front's leader Abu Mohammed al-Jolani pledged allegiance last month to Al Qaeda leader Ayman al-Zawahiri.
Al-Nusra Front is reputed to be among the fiercest and best equipped rebel groups fighting the regime.
The group has claimed most suicide attacks to have struck the country during the course of the conflict, which broke out when a peaceful movement for change morphed into a bloody civil war after the regime unleashed a brutal crackdown against dissent.