BEIRUT — A video that appears to show a unit of Syrian rebels kicking terrified, captured soldiers and then executing them with machine guns raised concerns Friday about rebel brutality at a time when the United States is making its strongest push yet to forge an opposition movement with which it can work.
UN officials and human rights groups believe President Bashar Assad’s regime is responsible for the bulk of suspected war crimes in Syria’s 19-month-old conflict, which began as a largely peaceful uprising but has transformed into a brutal civil war.
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,’’ said Suzanne Nossel, head of Amnesty International.
The United States has called for a major leadership shakeup 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 of opposition groups to support.
‘‘We condemn human rights violations by any party,’’ State Department spokeswoman Victoria Nuland said. ‘‘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 faction was involved, though the 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, 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 gunman 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. Moments later, gunfire erupts for about 35 seconds, screams are heard, and the men 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 that day.
Amnesty International’s forensics analysts did not detect signs of forgery in the video, according to Nossel. The group has not been able to confirm the location, date, or identity of those in the footage, she said.
After the assault Thursday, rebels took full control of Saraqeb, a strategic position on the main highway linking Syria’s largest city, Aleppo — which rebels have been trying to capture for months — with the regime stronghold of Latakia on the Mediterranean coast.
On Friday, at least 143 people, including 48 government soldiers, were killed in gunbattles, regime shelling attacks on rebel-held areas and other violence, the Observatory said.
Of the more than 36,000 killed so far in Syria, about one-fourth are regime soldiers, according to the Observatory. The rest include civilians and rebel fighters, but the group does not offer a breakdown.