A spokesman for the UN High Commissioner for Human Rights says a video showing what appears to be Syrian rebels executing government soldiers could be evidence of war crimes.
An amateur video posted on YouTube appears to show Syrian rebels beating around 10 captured men.
The soldiers were forced to lie on the ground and were shot dead.
The UN spokesman, Rupert Colville, said in Geneva that the video must be verified.
"We need to examine this carefully. It will be examined carefully," he said. "But the allegations are that these were soldiers who were no longer combatants. And therefore, at this point, it looks very likely that this is a war crime."
Amnesty International and the British-based Syrian Observatory for human rights both condemned the apparent executions.
Amnesty International said the footage "depicts a potential war crime in progress and demonstrates an utter disregard for international humanitarian law by the armed group in question."
Colville said if verified, the video could be used in legal proceedings against the perpetrators.
"And once again, we call on all parties to the conflict in Syria to respect international human rights law and international humanitarian law, and there should be really no illusions that accountability will follow," he said. "There is a lot of evidence for many of these crimes that have been taking place. This video -- if it's verified, if the details become a little clearer -- could well be part of that evidence."
The Syrian Observatory for Human Rights said on November 1 that rebels fighting against President Bashar al-Assad's regime attacked three army checkpoints in the country's northwest, killing 28 soldiers.
No group has claimed responsibility for those attacks in Idlib Province.
Fighting in Idlib intensified after rebels seized the town of Maaret al-Numan last month.
This week, U.S. Secretary of State Hillary Clinton voiced Washington's frustration with the inability of the opposition Syrian National Council (SNC) to unite the resistance against Assad's regime.
Clinton called for a new, "larger" opposition to include those who are fighting on the front lines in Syria.
She has also warned that radical Islamists are trying to "hijack" the Syrian uprising.
Activists estimate more than 35,000 people have been killed since the uprising against Assad's regime began in March 2011.
With reporting by Reuters, AP, and AFP