A video showing soldiers being executed by rebels "very likely" shows a war crime, the UN human rights body said, and the main opposition Syrian National Council called for those responsible for the executions to be called to account.
Two days ahead of key talks among the opposition in Qatar, the SNC lashed out at accusations from Washington that it was not fully representative of the country's diverse dissident forces.
"Any discussions aimed at passing over the Syrian National Council or at creating new bodies to replace it are an attempt to undermine the Syrian revolution by sowing the seeds of division," the SNC said.
The United States denied that it was trying to control the make-up of Syria's next leadership, insisting it was simply seeking to ensure that more voices were heard.
US Secretary of State Hillary Clinton this week voiced frustration with the SNC, saying it was not representative of on-the-ground opposition forces and that it "can no longer be viewed as the visible leader of the opposition".
Reports have emerged that Washington will push in Qatar for an overhaul of the opposition, with long-time dissident Riad Seif touted as the potential head of a new government-in-exile dubbed the Syrian National Initiative.
Washington has accused the SNC of failing to unite the resistance to President Bashar al-Assad beyond a small group of exiles, of not representing Syria's ethnic make-up and of alienating activists and rebel forces inside the country. But on Friday State Department spokeswoman Victoria Nuland said, "This is not a matter of the US dictating".
Nuland said the United States had backed the SNC for more than a year but now felt it needed to broaden its political base to "connect the various political groups inside Syria" fighting Assad's regime.
Washington, she said, wants to see a broader spectrum of communities in the opposition leadership including "not only the Sunni population but the Alawis, the Druze, the Christians, the Kurds, any other minority groups, women".
The Russian foreign ministry said Clinton's comments on the SNC clashed with the agreements world powers reached on the conflict in Geneva in June, accusing Washington of trying to settle Syria's conflict "exclusively on their terms".
The rebels faced growing criticism on Friday after the video was posted on YouTube, appearing to show opposition fighters beating and executing soldiers after attacks on Thursday on checkpoints near the northwestern town of Saraqeb.
The video -- the authenticity of which could not be verified -- showed about 10 soldiers being beaten, then lined up on the ground and executed with automatic rifles.
The SNC called for those responsible to be held accountable. "We urge the (rebel) Free Syrian Army and the revolutionary movement on the ground to hold to account anyone who violates human rights," SNC human rights committee head Radif Mustafa told AFP.
The UN human rights body said the video appeared to show a war crime and warned that "accountability will follow" for those who commit atrocities. "It is very likely that this was a war crime, another one," Rupert Colville, spokesman for UN High Commissioner for Human Rights Navi Pillay, told reporters in Geneva. "We call on all parties to the conflict in Syria to respect international law," he said.
US State Department spokeswoman Nuland told journalists: "There is no justification for that kind of behaviour ever. Anyone committing atrocities should be held to account."
Britain said it was "deeply concerned" by the video, with a Foreign Office spokesman saying: "We condemn atrocities committed by the armed opposition in Syria just as we condemn the many atrocities carried out by the regime."
A French foreign ministry spokesman said: "Our position is clear: to battle impunity, whatever the source."
Rebels were meanwhile reported to have seized a strategic crossroads in the northwest, in a move that will further limit the regime's ability to reinforce its troops in the northern commercial hub of Aleppo.
Rebel attacks forced troops to withdraw from their last position in the Saraqeb area, where the roads to Aleppo from Damascus and from the Mediterranean coast meet, the Syrian Observatory for Human Rights said.
After also seizing three checkpoints nearby on Thursday, the rebels now control an area extending 25 kilometres (15 miles) in all directions from Saraqeb, the Britain-based watchdog said.
At least 139 people, including 44 civilians, were killed across Syria on Friday, said the Observatory, which relies when compiling its tolls on a countrywide network of activists and medics in civilian and military hospitals.
It says more than 36,000 people have been killed since the uprising against Assad's rule broke out in March 2011 as a protest movement inspired by the Arab Spring before escalating into an armed rebellion.
Thousands meanwhile joined traditional Friday protests against the regime.
In the northwestern town of Kfar Nabal, protesters contrasted the toll of hurricane Sandy in the United States with that of the uprising, using the name of Assad's mother on a sign reading: "Sandy: 90 victims -- Anissa: 40,000 victims."