BEIRUT — 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, a pro-opposition monitoring group said.

The Britain-based Syrian Observatory for Human Rights said the death toll could rise as high as 80 in what it called a "new massacre." It was not clear who carried out the killings.

Photos posted online by activists showed the muddied bodies of about a dozen men lying by a small river in what they said was the Bustan al-Qasr neighbourhood of Aleppo.

Close-up shots of some of the corpses showed they had what appeared to be gunshot wounds to the head.

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 have been stuck in a stalemate with government forces. The city is divided roughly in half between the two sides.