BEIRUT (Reuters) - Armed rebels in Syria have kidnapped, tortured and executed members of the security forces and supporters of President Bashar al-Assad, Human Rights Watch (HRW) said on Tuesday.
The New York-based advocacy group condemned the tactics by opposition fighters, who have long accused government troops and loyalists of carrying out similar abuses.
"The Syrian government's brutal tactics cannot justify abuses by armed opposition groups," said Sarah Leah Whitson, HRW Middle East director, in an open letter to dissident groups including the official opposition body the Syrian National Council (SNC).
"Opposition leaders should make it clear to their followers that they must not torture, kidnap or execute under any circumstances," she added.
Washington said it would "absolutely denounce" human rights violations by the rebels, but said most of the abuse was being carried out by pro-Assad forces.
"Within the context of what's happening in Syria, we cannot lose sight of the fact that the great preponderance of violence against civilians and innocent people is being perpetrated by forces under the control of the Assad regime," said White House spokesman Jay Carney.
Syria's armed opposition is highly fragmented and many militias do not appear to belong to an organised command structure or to be following SNC orders.
The year-long uprising in Syria, in which the United Nations says more than 8,000 people have been killed, started as a peaceful protest movement. But it has become increasingly violent, with daily clashes between rebels and security forces around the country.
HRW cited dozens of YouTube videos in which Syrian security forces or their alleged supporters confessed to crimes, apparently under duress. At least 18 of the videos showed detainees who were bruised, bleeding, or suffering from other signs of physical abuse, the group said.
One video showed a man hanging by his neck from a tree in front of armed fighters, with commentary indicating he was a member of the Shabbiha, feared irregular forces loyal to Assad.
HRW said some of the attacks appeared to have targeted Shi'ite Muslims or members of Assad's own Alawite sect.
Analysts warn the uprising could degenerate into civil war, pitting Alawites against Sunni Muslims, who make up 75 percent of the 23-million population.
That it turn would add to strains along the Middle East's sectarian divide, with Assad's backer Iran, and Tehran's Shi'ite allies in Lebanon and Iraq against the Sunni powers which dominate Arab governments from Egypt to the Gulf.
HRW's Whitson said: "(The Syrian Opposition) need to make it clear that they envision a Syria that turns the page on Assad-era violations and welcomes all - regardless of their religious group or background - without discrimination."
Additional reporting by Matt Spetalnick in Washington; Editing by Robin Pomeroy