The bodies of 78 young men, all executed with a single gunshot, were found Tuesday in a river in Aleppo city, adding to the grim list of massacres committed during Syria's 22-month conflict, AFP reported.
The gruesome discovery came ahead of a briefing by peace envoy Lakhdar Brahimi to the UN Security Council on the uprising, which the United Nations says has left more than 60,000 people dead.
Abu Seif, a rebel fighter, told AFP 78 bodies were retrieved from the Quweiq River and that 30 more were still in the waters but out of reach because of regime snipers.
"The regime threw them into the river so that they would arrive in an area under our control, so the people would think we killed them," he said.
But a security official accused "terrorists," the regime term for the rebels, of the killings, adding the victims were residents kidnapped from the opposition-held district of Bustan al-Qasr.
Their families had tried to negotiate their release before they were killed overnight, he said.
There was no immediate claim of responsibility, but the official SANA news agency said the jihadist Al-Nusra Front carried out the executions.
"Terrorist groups from Al-Nusra Front in Aleppo carried out a mass execution of dozens of abducted people and threw their bodies in the Quweiq River," the agency said.
Al-Nusra Front, a rebel group which has a strong presence on the ground in northwestern Syria, has been blacklisted by the United States as a "terrorist" organization.
Al-Nusra is an affiliate of Al Qaeda, believing in the hope of reviving the Islamic Caliphate that will build a Muslim Empire to eventually rule the world.
The group is one of 13 factions in the radical Islamist rebel council that announced its secession from the main opposition force several weeks ago.
The Syrian Observatory for Human Rights gave a toll of 65 bodies but warned the figure could rise significantly.
"They are in their 20s and were executed by a bullet to the head. Most of them had their hands tied behind their backs and were in civilian clothes," said the watchdog, according to AFP.
The scene on the banks of the Quweiq was grim, as muddied corpses are dredged out and hundreds of distressed people flocked around to see if they could spot among the bodies a father, a brother, a son or a husband.
Volunteers helped place the bodies on a truck. They were then taken to a school where they were laid out and covered, reported AFP.
Meanwhile, violence raged elsewhere in Aleppo province, where seven children were killed in air strikes on the town of Safireh, the Observatory said, giving a toll of 91 people killed across Syria on Tuesday.
In Damascus a member of parliament was seriously injured when a explosive device strapped to his car exploded, the Observatory said.
The bloodshed came as rebels captured a vital bridge across the Euphrates River in Deir Ezzor city, largely severing an army supply route to Hasakeh province further north.
The nearby regime security headquarters and a smaller bridge were also captured, prompting retaliatory air strikes on the critical crossings.
A few days ago, regime warplanes launched a wave of deadly raids across Syria, including one that killed eight children.
Last Monday, a suicide car bombing in central Syria killed at least 30 people.