Activists said that at least 65 people, apparently shot in the head, were found dead with their hands bound in a district of the northern Syrian city of Aleppo on Tuesday (January 29).
The British-based Syrian Observatory for Human Rights, which says it provides objective information about casualties on both sides of Syria's war from a network of monitors, said the death toll could rise as high as 80. It was not clear who had carried out the killings.
Opposition activists posted a video which appears to be showing a man, filming at least 51 muddied male bodies alongside what they said was the Queiq River in the rebel-held Bustan al-Qasr neighborhood of Aleppo.
The bodies had gunshot wounds to their heads, and their hands were bound. Blood was seeping from their heads and some of them appeared to be young, possibly teenagers, and dressed in jeans, shirts and sneakers.
The Queiq River rises in Turkey and travels through government-held districts of Aleppo before it reaches Bustan al-Qasr.
It is hard for Reuters to verify such reports from inside Syria because of restrictions on independent media.
Government forces and rebels in Syria have both been accused by human rights groups of carrying out summary executions in the 22-month-old conflict, which has claimed more than 60,000 lives.
Rebels pushed into Aleppo, Syria's most populous city, over the summer, but are stuck in a stalemate with government forces. The city is divided roughly in half between the two sides.
The revolt started as a peaceful protest movement against more than four decades of rule by President Bashar al-Assad and his family, but turned into an armed rebellion after a government crackdown.
More than 700,000 people have fled, the United Nations says.