Two parties drawing their support from the majority Sinhalese community said they would not support the UN inquiry, set up to investigate widespread allegations of rights abuses and mass killings of civilians as the Tamil separatist war drew to a bloody close in 2009.
Sinhalese opposition parties oppose any foreign probe into the conflict and the government has vowed not to assist the UN investigation, which was set up this week in line with a US-initiated resolution passed in March.
"We do not approve this UN investigation, but we want the government to set up a domestic mechanism to go into the recent killings," said JVP leader Anura Dissanayake, adding that the local investigation should probe ongoing human rights abuses as well as alleged war crimes.
Rights groups have accused government forces and police of targeting dissidents despite the end of the war.
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Sri Lanka's parliament will debate the proposed UN investigation over two days and take a vote on Wednesday evening, deputy Speaker Chandima Kodikara told reporters Friday.
International rights groups say up to 40,000 civilians were killed by troops in the final stages of the war, which ended with the elimination of the top Tamil rebel leadership.
Colombo strongly denies that its troops killed any civilians, but instead says the defeated Tamil Tiger guerrillas used civilians as a human shield. Rights groups have accused both sides of targeting civilians.