Amateur video posted on You Tube on Monday showed images of 20 dead Syrian soldiers, blindfolded and handcuffed, after they were apparently executed in the northern city of Aleppo.
Two videos showed the dead men dressed in army fatigues and kneeling in a long line along a road with their bloodied heads lying on the pavement.
It was not possible to verify the authenticity of the videos.
Rami Abdulrahman, head of the British-based opposition watchdog the Syrian Observatory for Human Rights, said that four people in Aleppo had told him of the incident which happened on either Friday or Saturday.
"The soldiers were from the Aleppo district of Sekenat Hanano but they were killed in the district of Sabaa Baharat," he said by telephone.
The videos showed rebel fighters holding assault rifles standing around the dead men, calling them "(President Bashar al-) Assad's Dogs."
"The Suleiman al-Farisi brigade ... killed several members from the (state) security," a man said off camera, filming a car with the name of the brigade written on the bonnet.
The Suleiman al-Farisi brigade, from the northern town of al-Bab, is one of several units from Aleppo province that have taken up arms against Assad and pushed into the region's capital, Syria's commercial hub and most populous city, in July.
Navi Pillay, the U.N. High Commissioner for Human Rights blamed on Monday both sides in the Syrian conflict for human rights violations and said that justice would eventually catch up with them.
The Syrian Observatory for Human Rights says more than 27,300 people have died in an uprising to topple Assad that has lasted more than 17 months, including about 19,500 civilians and rebel fighters, 1,100 army deserters and 6,700 soldiers and members of the security forces.