The video from Syria shows a man being thrown out a window, apparently to his death. Behind the camera, someone says, "This is the fate of all traitors."
It is one of several videos that Amnesty International has seen in recent weeks purporting to show Syrian dissidents executing captured members of the Syrian security forces and their suspected allies, a trend that the human rights group says it is investigating.
As the uprising against Syrian President Bashar Assad has raged for nearly a year and a half, human rights groups have excoriated the Syrian government and the militias that back it for carrying out unlawful killings.
In an earlier report, Amnesty International found that dozens of towns believed to back the rebels had suffered revenge attacks in which men were dragged from their homes, killed and burned. The vast majority of abuses in the country were carried out by Syrian government forces or its allies, the group said.
Opposition activists said another such killing was carried out in the Damascus neighborhood of Mezze this week, where they said 19 unarmed men and a child were found dead. Although Amnesty International could not confirm allegations that government forces killed the men because they were suspected of helping rebels, it said the deaths mirrored the pattern of government killings it had documented in Syria.
But as the bloody conflict has worn on, evolving from a protest movement into an armed rebellion, the human rights group says it has received increasing reports of armed opposition groups engaging in similar abuses.
Videos, testimony and media reports indicate that dozens of people suspected of working for or helping the Syrian government might have been executed, Amnesty International said Wednesday.
"The leadership of all sides must make it clear that they will not tolerate such abuses by anyone under their command," said Ann Harrison, deputy director of its Middle East and North Africa program.
Other reports of killings have surfaced: When rebels took over border crossings between Iraq and Syria, Iraq's Deputy Interior Minister Adnan Assadi told the Agence France-Presse news agency that Iraqi border guards had witnessed the Free Syrian Army executing 22 Syrian soldiers.
The reported killings only add to worries about the Syrian opposition, a fragmented and decentralized movement that could end up governing Syria if it were to topple Assad. U.S. officials have been especially concerned that some of the armed groups might be sympathetic to Al Qaeda.
The United States has warned against such abuses.
A future Syria "has to be a Syria that protects the human rights, the dignity, the democratic opportunity for all ... anybody claiming to fight on behalf of the Syrian people needs to be operating in that direction," State Department spokeswoman Victoria Nuland told reporters Tuesday.
-- Emily Alpert in Los Angeles