Seconds later, amid the screams of those captured, gunfire erupts for around 35 seconds and the men on the floor are seen shaking and twitching, apparently from being struck by bullets. The spray of bullets raises a cloud of dust from the ground.
The video is titled "prisoners and dead from the regime military at the Hmeisho checkpoint." On Thursday, the Observatory had reported 12 soldiers killed at Hmeisho, outside Saraqeb, one of three major rebel attacks on military checkpoints in the area.
Thursday saw heavy casualties for the military around the country, with 83 soldiers killed in attacks by rebels and clashes, the Observatory said. Half of those were killed in Idlib province, where Saraqeb is located.
Ausama Monajed, a Britain-based member of the Syrian National Council opposition group, called for an investigation of the incident. He said the unit responsible for the apparent killing of unarmed regime soldiers must be tracked down and tried.
Monajed alleged that the regime has created "fake" rebel units in the past to commit atrocities and tarnish the rebels' reputation.
"We are not saying this is the case," he said. "We need to identify the unit."
He said atrocities committed by rebels are relatively rare compared to what he said was the "massive genocide by the regime."
Rami Abdul-Rahman, who heads the Observatory, estimated that around 20 people were killed in the portrayed summary executions, but the number could not be accurately verified.
The identity or affiliation of the fighters on video was unknown. But Abdul-Rahman and other activists said several brigades are known to operate in the area, including Jabhat al-Nusra, a shadowy Islamic militant group with an al-Qaida-style ideology.
At least 36,000 people have been killed since the uprising against Assad began in March 2011 and then transformed into an outright civil war, according to anti-regime activists. Thousands of people have been killed over the past few months, including more than 500 last week during a four-day internationally-brokered cease-fire that collapsed shortly after it went into effect.