Beirut - The bodies of 147 men pulled out of a river in Aleppo between January and March were "probably" executed in government-controlled areas of Syria's main northern city, Human Rights Watch said on Tuesday.
HRW said it had visited the war-torn city and interviewed the residents and activists who found the bodies, a forensics expert and 18 of the bereaved families.
The New York-based watchdog also reviewed 350 photographs and videos of the victims as part of their investigation.
"Many of the victims bore signs of having been detained and then executed, such as hands tied behind their back, gunshot wounds to their head, and tape across their mouth," the report said.
HRW said that, although it has yet to finish its investigation into who carried out the killings, the location the bodies were found in and the victims' last reported whereabouts suggested they were killed in government-controlled areas.
"The bodies floating down Aleppo's river tell a grisly tale," HRW researcher Ole Solvang said.
Scene of intense fighting
"It's hard to see how 147 people could have been executed and their bodies flung in the river in government-controlled territory, as the evidence indicates, without the knowledge of government forces operating in the area," he added.
Activists in the area told HRW researchers that they had fished 230 bodies out of the river between 29 January and 14 March, but the rights group itself was able to identify 147 victims, all male and aged between 11 and 64.
Aleppo has been the scene of intense fighting since rebels seized a swathe of districts in July last year.
The river, which runs from north to south, crosses the battle lines that divide the government-controlled northwest from the rebel-held southeast.
In January, after the first bodies were found, the official SANA news agency reported that the foreign ministry had written to the UN accusing al-Qaeda loyalists of the al-Nusra Front of carrying out the killings.
HRW said the report gave no other explanation for the allegation than that the bodies were recovered in rebel-controlled areas.
"These executions underline the urgent need for objective and impartial investigations," Solvang said.
"The UN Security Council should refer Syria to the International Criminal Court and demand that the Syrian government provides the UN Commission of Inquiry access to the country."