But investigators of human-rights abuses say rebel atrocities are on the rise.
At this stage "there may not be anybody with entirely clean hands," Suzanne Nossel, head of the rights group Amnesty International, told the Associated Press.
The United States has called for a major leadership shake-up of Syria's political opposition during a crucial conference next week in Qatar. Washington and its allies have been reluctant to give stronger backing to the largely Turkey-based opposition, viewing it as ineffective, fractured and out of touch with fighters trying to topple Assad.
But the new video adds to growing concerns about those fighters and could complicate Washington's efforts to decide which of the myriad opposition groups to support.
"We condemn human-rights violations by any party," U.S. State Department spokeswoman Victoria Nuland said, commenting on the video. "Anyone committing atrocities should be held to account."
She said the Free Syrian Army has urged its fighters to adhere to a code of conduct it established in August, reflecting international rules of war.
The summary execution of the captured soldiers, purportedly shown in an amateur video, took place Thursday during a rebel assault on the strategic northern town of Saraqeb, said the Britain-based Syrian Observatory for Human Rights, an activist group.
It was unclear which rebel faction was involved, though the al-Qaeda-inspired Jabhat al-Nusra was among those fighting in the area, the Observatory said.
The video, posted on YouTube, shows a crowd of gunmen in what appears to be a building under construction. They surround a group of captured men on the ground, some on their bellies as if ordered to lie down, others sprawled as if wounded. Some of the captives are in Syrian military uniforms.
"These are Assad's dogs," one of the gunmen is heard saying of those cowering on the ground.
The gunmen kick and beat some of the men. One gunman shouts, "Damn you!" The exact number of soldiers in the video is not clear, but there appear to be about 10 of them.
Moments later, gunfire erupts for about 35 seconds, screams are heard and the men on the floor are seen shaking and twitching. The spray of bullets kicks up dust from the ground.
The video's title says it shows dead and captive soldiers at the Hmeisho checkpoint. The Observatory said 12 soldiers were killed Thursday at the checkpoint, one of three regime positions near Saraqeb attacked by the rebels in the area that day.
Amnesty International's forensics analysts did not detect signs of forgery in the video, according to Nossel. The group has not yet been able to confirm the location, date and the identity of those shown in the footage, she said.