At least 65 people, apparently shot in the head, were found dead with their hands bound in a district of the northern Syrian city of Aleppo on Tuesday, activists said.
The British-based Syrian Observatory for Human Rights, which says it provides objective information about casualties on both sides of Syria's war from a network of monitors, said the death toll could rise as high as 80. It was not clear who had carried out the killings.
Opposition activists posted a video of a man filming at least 51 muddied male bodies alongside what they said was the Queiq River in the rebel-held Bustan al-Qasr neighborhood of Aleppo.
The bodies had gunshot wounds to their heads, and their hands were bound. Blood was seeping from their heads and some of them appeared to be young, possibly teenagers, and dressed in jeans, shirts and sneakers.
The Queiq River rises in Turkey and travels through government-held districts of Aleppo before it reaches Bustan al-Qasr.
"They were killed only because they are Muslims," said a bearded man in another video said to have been filmed in central Bustan al-Qasr after the bodies were removed from the river. A pickup truck with a pile of corpses was parked behind him.
It is hard for Reuters to verify such reports from inside Syria because of restrictions on independent media.
Government forces and rebels in Syria have both been accused by human rights groups of carrying out summary executions in the 22-month-old conflict, which has claimed more than 60,000 lives.
Rebels pushed into Aleppo, Syria's most populous city, over the summer, but are stuck in a stalemate with government forces. The city is divided roughly in half between the two sides.
The revolt started as a peaceful protest movement against more than four decades of rule by President Bashar al-Assad and his family, but turned into an armed rebellion after a government crackdown.
More than 700,000 people have fled, the United Nations says.
REBELS FIGHT KURDS
In the eastern city of Deir al-Zor, insurgents including al Qaeda-linked Islamist fighters captured a security agency after days of heavy fighting, according to an activist video issued on Tuesday.
The fighters freed prisoners from the building, it added.
The video, posted online, showed men armed with assault rifles cheering as they stood outside a building that they said was a local branch of Syria's intelligence agency.
Some of the fighters carried a black flag with the Islamic declaration of faith and the name of the al-Nusra Front, which has ties to al Qaeda in neighboring Iraq. The video also showed tanks, which appeared to be damaged, and a room containing weapons.
The war has become heavily sectarian, with rebels who mostly come from the Sunni Muslim majority fighting an army whose top generals are mostly from Assad's Alawite sect, an offshoot of Shi'ite Islam. Assad has framed the revolt as a foreign-backed conspiracy and blames the West and Sunni Gulf states.
Fighting also took place in the northern town of Ras al-Ain, on the border with Turkey, between rebels and Kurdish militants, the Observatory said.
The insurgents have been battling fighters of the Kurdish People's Defence Units for about two weeks in the area, and scores of people have died in the violence.