The United Nations Organization says a video that appears to show Syrian rebels killing soldiers or pro-government militiamen could be evidence of a war crime.
The footage shows gunmen beating a group of prisoners cowering on the floor before opening fire at them.
It has been alleged that Islamist militants carried out the attack after seizing army checkpoints on Thursday.
Unconfirmed reports say troops have now quit all bases near the strategic northern town of Saraqeb.
The town lies near both the main Damascus-Aleppo highway and the highway linking Aleppo to the coastal city of Latakia – making it doubly strategic.
The army, meanwhile, continued its air strikes across Syria on Thursday.
In all, more than 150 people reportedly died in fighting, said the Syrian Observatory for Human Rights (SOHR), a UK-based activist group.
The SOHR said that among the victims were more than 70 government soldiers, 43 civilians and 38 rebels.
The claim has not been independently verified.
In a separate development, Russian Foreign Minister Sergei Lavrov, international peace envoy Lakhdar Brahimi and Arab League Secretary General Nabil el-Araby would meet in Cairo on Sunday to discuss the Syrian crisis, the Arab League announced.
The alleged shootings took place after the rebels overran the strategic army checkpoints between on Thursday.
The video purportedly shows agitated rebels kicking and pushing the soldiers or pro-government militiamen, known locally as “shabiha”, to the ground inside one of the seized buildings. Shots are then seen fired into the cowering mass of bodies.
A spokesman for the UN High Commissioner for Human Rights, Navi Pillay, said it appeared that the victims “were no longer combatants and therefore, at this point, it looks very like a war crime”.
Spokesman Rupert Colville added that the video – if proved to be genuine – would almost certainly form part of a future prosecution.
Meanwhile, Amnesty said in a statement: “This shocking footage depicts a potential war crime in progress, and demonstrates an utter disregard for international humanitarian law by the armed group in question.”
No group has so far admitted carrying out the alleged killings.
However, a rebel fighter from Idlib province, Abu Abdul Rahim, told the Guardian that a Salafi-jihadist group was behind the killings, which he said had occurred in al-Nayrab, to the west of Saraqeb.
He said the Salafists of the Dawood brigade and Suqur al-Sham did not answer to any military council affiliated to the rebel Free Syrian Army.
For months, activists have reported similar summary executions by government forces virtually every day.
But there has been mounting evidence of similar tactics being used by some rebel groups too, although many have signed a code of practice banning such abuses.
US Secretary of State Hillary Clinton earlier warned that radical Islamist fighters were trying to hijack the Syrian revolution.
The comments have drawn an angry response from some opposition leaders, who say that it is the failure of the outside world to support the uprising with practical help that has left the field open to the radicals.
The SOHR more than 36,000 people have been killed since protests against President Bashar al-Assad erupted in March 2011.
The SOHR is one of the most prominent organizations documenting and reporting incidents and casualties in the Syrian conflict. It says its reports are impartial, though its information cannot be independently verified.
Recent alleged rebel atrocities
• 22 June – Damascus accuses ‘terrorists’ of killing 25 villagers in northern Syria and mutilating their bodies
• 6 July – footage shows a rebel questioning a soldier before shooting him – location unknown
• 1 August – four apparent Assad loyalists are seen put against the wall and shot in public in Aleppo
• 14 August – rebels are shown in a video throwing dead bodies of government snipers from an Aleppo roof
• 17 September – armed opposition groups are accused by Human Rights Watch of torturing and summarily executing detainees in Aleppo, Latakia, and Idlib