COLOMBO -- Sri Lanka's Tamil Tiger rebels are using a cease-fire in the island's ethnic war to murder political rivals with impunity, two international human rights groups said on Friday. The charges by Amnesty International and Human Rights Watch come several days after the U.S. State Department said the actions by the Tigers were undermining the peace process.

"Members of Tamil political parties are being gunned down and the available evidence points to the Tamil Tigers," Brad Adams, executive director of the Asia Division of Human Rights Watch, said in a statement.

The U.S. State Department said more than three dozen political opponents of the Tigers have been murdered while Amnesty International says at least 22 have been killed.

"The use of political assassinations and violence threatens to seriously undermine moves made toward establishing a just system of governance that will serve all citizens of Sri Lanka," Amnesty International said in a statement.

There was no immediate comment from the Tigers, who are blamed for the 1993 killing of Sri Lankan President Ranasinghe Premadasa and the suicide bombing of former Indian Prime Minister Rajiv Gandhi two years earlier.

The Tigers have denied targeting political opponents, but have admitted using child soldiers, Reuters reported.

The Tigers and government signed a cease-fire in February 2002, and although the rebels suspended peace talks in April, the peace bid is still seen as the best change of permanently ending the war that has killed 64,000.

On Wednesday, U.S. State Department Deputy Spokesman Philip T. Reeker criticized the Tigers for "undermining confidence in the peace process" by maintaining a camp truce monitors say is in violation of the cease-fire.

He said the United States called on the Tigers to "renounce terrorism and cease terrorist acts, including political assassinations, and to comply with the terms of the cease-fire agreement they signed".

The rebels have been fighting for a separate Tamil state in the north and east but said during earlier rounds of talks they were willing to settle for a political solution.