Human Rights Watch said it interviewed witnesses and visited the site where bodies were discovered in Aleppo. Based on photographs and other evidence, the organization said many of the victims had their hands tied behind their backs, tape over their mouths or other signs of a summary execution.
The organization said "at least 147" of the victims were likely executed in government-controlled areas.
Human Rights Watch stressed its investigation wasn't definitive in terms of blame. Emergencies researcher Ole Solvang said in a statement Tuesday the bodies "tell a grisly tale."
"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 said from London.
The report follows an assessment from an international Commission of Inquiry on Syria that said civil war "has reached new levels of brutality."
The 29-page report by the U.N.-backed commission, published Tuesday, said pro-government forces likely committed murder, torture and other inhumane acts during the war. The report said anti-government forces were a threat to the civilian population and were also responsible for war crimes, including torture.
The International Committee of the Red Cross said Monday at least 80,000 people have died during the Syrian war, 1.6 million people are displaced and 7 million require some form of humanitarian assistance.