Rebel groups in Aleppo say they have recovered more than 100 bodies from a small river in the south of the city and say more are yet to be dragged from the water after what appears to be one of the biggest massacres to have taken place in the war-ravaged city.

A video posted online today showed rows of bodies, most of them men in their 20s and 30s, with their hands tied behind their backs and evident bullet wounds to their heads. The graphic footage showed blood seeping from the corpses across silt left on the watercourse's concrete bank by floodwaters that have recently receded.

One witness at the scene said he counted 108 bodies and rebel groups said they expected the final death toll to climb to as many as 120, though the Guardian was unable to independently verify the number of dead. The corpses were found on Tuesday morning in Bustan al-Qasr, in the southern city centre. The site is near one of several frontlines that divides opposition groups in the east and south of Aleppo from loyalist forces in the north.

Where the men were executed is not yet clear. Winter rain has given the river a strong current in recent weeks and initial assessments suggested that the bodies could have been carried some distance.

The Syrian regime blamed "terrorist gangs" for the killings, a term it uses for all groups fighting against it. A regime official in Damascus suggested that rebel groups were now leading a recovery in an attempt to "cover for their crimes".

A Syrian official told Agence France Presse: "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."

Rebel groups, however, blamed the Syrian government for the killings, suggesting that some of the victims had crossed into loyalist areas then disappeared. None of the victims had identification cards.

Residents of rebel-held eastern Aleppo have reportedly arrived at the site where the bodies were found to try to identify missing relatives. The remains were loaded into trucks and taken to the nearby Zarzur Hospital, where rebel officials were trying to organise an identification process.

Credit: Guardian graphics

A resident of rebel-held eastern Aleppo who called himself Omar al-Halabi told the Guardian that 11 bodies had been identified by nightfall and claimed by their families for burial.

"Their relatives said that they live in ... areas under the control of the FSA like Ansari, Sukari and Al-Bustan districts," he said. "None were members of the Free Syria Army [FSA].

"I spoke to a guy who was able to identify his brother and his friend's bodies," he said. "[He] said that his brother and his friend were heading to one of the districts which are still under the control of the Syrian army in Aleppo and they did not come back three days ago. These two men were in their fifties.

"Other bodies were identified by their relatives who said that they had been detained at the Air Force Intelligence headquarters in Aleppo 10 days ago."

An eyewitness to the recovery effort said several bullets were fired during the day from buildings to the north.

Extrajudicial killings have become increasingly frequent in many parts of Syria over the past six months. However, Tuesday's death toll in Aleppo is believed to be without precedent in the city since rebels launched an assault there in July.

In the months since then, the opposition has entrenched itself in the east of the city and the regime has hunkered down in the north-west. The south-western suburbs of Saif al-Dawli and Salahedin remain battleground areas. Few residents cross the divide, which is manned on one side by Syrian soldiers and sniper positions and on the other by rebel groups.

As fighting and atrocities continue, Syrian citizens are leaving the country in increasing numbers. Over the past six weeks, an extra 200,000 refugees have sought shelter in Lebanon, Turkey, Jordan and Iraq, the UN High Commission for Refugees said on Tuesday.

More than 700,000 people have now fled Syria, the UN said, with 21,000 crossing into Jordan in the past week alone – about five times the number seen in previous weeks.

The UN has appealed for $1.5bn (£950m) in aid to fund shelter, food and education for Syrian refugees but says it has so far been able to secure only 3% of that figure from international donors.

With the violence in Syria showing no sign of abating, aid needs are likely to intensify in the first half of the year at least. At least another 1.5 million Syrians are believed to be internally displaced.

While some food aid, as well as homegrown fresh produce, has been delivered to battle zones such as Aleppo in recent weeks, there is no guarantee that supply lines can continue to be secured.

Fighting erupts near flashpoint areas of Aleppo most days. The site where the bodies were found is one such area, considered a no-go zone for the few remaining locals.