The bodies of at least 80 young men and boys, all executed with a single bullet to the head or neck, have been found in a river in the Syrian city of Aleppo, according to a watchdog as well as rebels.

The London-based Syrian Observatory for Human Rights said at least 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 fighter at the scene said the death toll is higher, pointing out that many more bodies were still being dragged from the water, in a rebel-held area.

"Until now we have recovered 68 bodies, some of them just teens," said Captain Abu Sada, adding that all of them had been "executed by the regime."

"But there must be more than 100. There are still many in the water, and we are trying to recover them."

A senior government security source said many of the victims were from Bustan al-Qasr and had been reported kidnapped earlier.

He accused "terrorists" of carrying out the executions and spreading propaganda to deflect responsibility.

"They were kidnapped by terrorist groups, who some are accusing of being pro-regime, and executed last night in a park in Bustan al-Qasr under their control," the source said by telephone.

"Now these terrorist groups are creating a media campaign, showing the bodies being recovered from the Quweiq River in an area under their control."

Failed negotiation attemtps

The source said 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.

"We will disclose the identities of those killed as soon as we are able to secure the bodies, which is a difficult process since the area is in the hands of terrorist groups".

A volunteer said as he helped load one of the bodies on a truck: "We don't know who they are because there was no ID on them.

Abu Sada said they would be taken to the hospital at Zarzur where relatives could seek to identify them.

"Those who are not identified will be buried in a common grave," noting that some were unrecognisable because of the impact of the bullet.

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.

"They could have been executed a couple of days ago and the current brought the bodies this far," an FSA fighter, Abu Anas, said.

The 129km 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," he says numbly, as he examines the body of a boy of about 12 with a bullet wound to the back of the neck.

"The shabiha [pro-government militia] seize people crossing the checkpoint ... and they torture and execute many of them," said Abu Anas.

All look to be young men, some teens, wearing jeans, button-up shirts and sneakers.