BEIRUT — Syrian rebels executed three brothers — one of them a rebel fighter — and a fourth relative, after accusing them of cooperating with President Bashar al-Assad's forces and killing a fellow rebel, a video posted on the Internet appears to show.

The video showed the four men sitting on chairs while a voice behind the camera says they "have sold their souls to the oppressor regime . . . and conspired against their brothers in religion and country". It later shows gunmen unleashing volleys of shots at four prone bodies.

Human rights groups have accused both sides in Syria's 22-month-old conflict of committing abuses — including summary executions — though they have blamed Assad's forces for most of the violations.

The Syrian Observatory for Human Rights said the four men were executed late last month after two days' of interrogation in the northern city of Aleppo.

The men are asked to introduce themselves and "confess" to their crimes, and respond in low and weak voices. Each one describes in few words how they killed a rebel after luring him into their house and then stabbing and shooting him.

"How do you describe the man you killed?" the voice behind the camera asks a young man who says his name is Omar, a fighter with Liwaa al Fatah. "He was a good man, our relation was good," he replies.

The second man says he was cooperating with security forces to arrest rebels and hand them over to government forces in return for a state job, adding that taking part in the killing was "his first job."

Another video uploaded on Monday showed a group of men purportedly from a rebel group called Al-Muhajereen wal Ansar battalion from the powerful Sukour Al-Sham Brigade, executing a man by slitting his throat, saying his death was punishment for "fighting God."