A video posted online appeared to show rebels in the Syrian city of Aleppo executing loyalist prisoners.
The video, uploaded to YouTube late Wednesday, shows four men, some apparently injured, being pushed into a courtyard by a crowd of armed men amid chants of "Allahu Akbar," or "God is greatest."
The cameraman moves to the back of the crowd as automatic gunshot rings out for more than 40 seconds. The shot then moves back to show the motionless bodies of the men.
The narrator of the video explains that the executed men were from the Barri clan, whom the rebels accuse of murder and siding with the government forces, according to a report by Al Jazeera.
A Human Rights Watch representative was quoted by the BBC as saying the act was potentially a war crime. The UK-based Syrian Observatory for Human Rights as labeling the apparent killings "criminal," the report said.
Government forces launched an offensive in Aleppo six days ago to recapture large areas controlled by rebels. The northern city near the border with Turkey is Syria's largest and its main commercial centre.
Activists said at least 90 people were killed on Wednesday across Syria, including 30 in attacks by government forces on the town of Artouz near Damascus.
UN peacekeeping chief Herve Ladsous was to brief the UN's Security Council on the Syrian situation Thursday, French Foreign Minister Laurent Fabius said.
Amnesty International on Wednesday released a report warning the international community to take steps to ensure any support for the rebels in Syria would not be used to commit human rights abuses, among other recommendations.
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