text: The footage shows gunmen beating and shooting a group of prisoners who were cowering on the floor.
  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 the main roads linking Aleppo to Damascus and the coastal city of Latakia.
  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 other developments:
  The alleged shootings took place after the rebels overran the strategic army checkpoints between on Thursday.
  The video appears to show 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 fired into the cowering mass of bodies.
  Rupert Colville, of the UN High Commission for Human Rights, said it appeared that the victims "were no longer combatants and therefore, at this point, it looks very like a war crime".
  He added that the video, if proved to be genuine, would almost certainly form part of a future prosecution.
  Rights group 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 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 killings 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, says the BBC's Jim Muir in neighbouring Lebanon.
  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 says 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 organisations documenting and reporting incidents and casualties in the Syrian conflict. It says its reports are impartial, though its information cannot be independently verified.
target: A video appearing to show Syrian rebels murdering soldiers or pro-government militiamen could be evidence of a war crime, the UN has said.