The bodies of at least 65 young men, all executed with a single gunshot to the head or neck, were found on Tuesday in a river in Aleppo city, adding to the grim list of massacres committed during Syria's 22-month conflict.
The gruesome discovery came as international envoy to Syria Lakhdar Brahimi was scheduled to brief the United Nations Security Council in New York.
The Syrian Observatory for Human Rights said 65 bodies were found in the Quweiq River, which separates the Bustan al-Qasr district from Ansari in the southwest of the city, but that the toll could rise significantly.
A Free Syrian Army officer at the scene, Captain Abu Sada, said at least 68 bodies had been recovered and that many more were still being dragged from the water, in a rebel-held area.
A senior government security source said many of the victims were from Bustan al-Qasr and had been reported kidnapped earlier.
He accused "terrorists," the standard regime term for people fighting to overthrow President Bashar al-Assad, of the killings and spreading propaganda to deflect responsibility.
"It has been confirmed that a number of the victims had been abducted by armed terrorist groups and their families had made repeated attempts to negotiate their releases.
An AFP correspondent saw at least 15 bodies already on a truck, which Abu Sada said would be taken to a hospital for identification.
Meanwhile, people were gathering at the bank seeking lost relatives.
"My brother disappeared weeks ago when he was crossing [through] the regime-held zone, and we don't know where he is or what has become of him," said Mohammed Abdel Aziz, as he looked at the mud-covered bodies one by one.
The 129-kilometre river originates in Turkey to the north and flows to the southwest of Aleppo, traversing both regime and rebel-held areas.
"This is not the first time that we have found the bodies of people executed, but so many, never," an FSA fighter, Abu Anas, said as he examined the body of a boy of about 12 with a gunshot wound to the back of the neck.
Opposition activists posted a video of a man filming at least 51 muddied male bodies alongside what they said was the Queiq River in the rebel-held Bustan al-Qasr neighbourhood 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.
"They were killed only because they are Muslims," said a bearded man in another video said to have been filmed in central Bustan al-Qasr after the bodies were removed from the river. A pickup truck with a pile of corpses was parked behind him.
It is hard for Reuters to verify such reports from inside Syria because of restrictions on independent media.
Elsewhere in Aleppo province, seven young children were among eight civilians killed in air strikes on the town of Safireh, the Observatory said, giving an initial toll of 37 people killed on Tuesday.
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.
"These gains in Deir Ezzor are very important because this strategic city is the gateway to a region rich in oil and gas resources," said Observatory director Rami Abdel Rahman.
The regime still controls Deir Ezzor military airport, but troops have abandoned the vast territory stretching from the city to the Iraqi border in the east, including the border town of Albu Kamal and its small military airbase.
"If the rebels continue to progress and gain control of what is left of military-held positions... it will be the first major city to fall into the hands of the rebels," Mr. Abdel Rahman said.
The advance came a day after Lebanese daily Al-Akhbar reported that Mr. al-Assad had said the army was gaining the lead.
The rebels' "playground is limited to some border areas with Turkey, Jordan and Lebanon, and some pockets in the countryside of the capital, which are being dealt with," Mr. al-Assad said, not mentioning the Iraqi border.
UN-Arab League envoy Brahimi is scheduled to brief the Security Council, where heavyweights Russia and the United States have found no common ground for a transition plan to end the brutal conflict.
With a report from Reuters