BEIRUT, Lebanon — Scores of muddied and waterlogged gunshot victims, most of them men in their 20s and 30s, were found dead in a suburb of Syria’s contested northern city of Aleppo on Tuesday. Insurgents and the government accused one another of carrying out the killings in what appeared to be the latest civil war atrocity.

Videos posted by opponents of President Bashar al-Assad seemed to show that many had been shot in the back of the head while their hands were bound. The Syrian Observatory for Human Rights, an activist organization based in Britain with a network of contacts in Syria, said at least 50 bodies had been located, scattered along the banks of a small river in the Bustan al-Qasr neighborhood, which is mostly under rebel control. Later reports put the tally much higher.

Al Jazeera quoted a commander from the insurgent Free Syrian Army, identified as Capt. Abu Sada, as saying that there could be more than 100 bodies, with many still submerged in the murky river, and that all had been “executed by the regime.”

Syria’s state news agency, SANA, later posted a report on its Web site that blamed the insurgent Islamist fighters of Al Nusra Front, and said the killings added to “a series of brutal massacres perpetrated by the terrorist groups against unarmed civilians.”