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.

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, said 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.

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.

The scene on the bank of the Quweiq was grim, as muddied corpses are dredged out of the water 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.

"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.

Volunteers helped place on a truck the bodies which were then taken to a school where they were laid out and covered in a blue cloth.

"We do not know who they are -- they were not carrying papers," a volunteer said as an AFP correspondent counted at least 15 bodies on one truck.

A number was placed next to each body and their faces were left uncovered to allow the identification by relatives at the school, where the nauseating stench of death lingers.

"There are those who drowned because the were shot in the legs or abdomen before being thrown into the water," said a nurse, noting some victims may have been killed as far back as three days ago.

The 129-kilometre (80 mile) 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," rebel 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.

"Jews would not have done this. Their only crime was that they were residents of Bustan al-Qasr and they were Sunni Muslims," a man cried out as he looked down at several of the bodies laying on the pavement waiting to be identified.

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.

And 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.

"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.

"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," said Abdel Rahman.

On the humanitarian front charity organisations on Tuesday pledged $182 million (RM560 million) for Syrians displaced at home or who have fled abroad because of the conflict on the eve of a donors conference in Kuwait.

US President Barack Obama announced an extra $155 million (RM477 million) to aid refugees fleeing what he said was "barbarism" propagated by the government of Syrian President Bashar al-Assad.