Sri Lanka warned the United Nations on Thursday against publishing a 'preposterous' report on alleged war crimes during the island's ethnic war, saying it could badly harm reconciliation efforts.

Foreign Minister G. L. Peiris asked UN Secretary General Ban Ki-moon not to release the study compiled by a panel of experts who looked into alleged rights abuses and crimes against humanity during fighting which ended in 2009.

The UN has said it intends to make the report public some time this week.

"The publication of this report will cause irreparable damage to the reconciliation efforts of Sri Lanka. It will damage the UN system too," Peiris told reporters in Colombo.

"This UN report is preposterous."

Peiris stressed that the government had never accepted Ban's appointment of the panel and warned him about initiating a full inquiry into suspected civilian deaths during the final stages of fighting.

He declined to comment on the contents of the report, a copy of which was handed over to him nine days ago, even though parts of the 200-page document were leaked in a pro-government newspaper last weekend.

"The consequences of publishing this report are far from favourable for the UN," Peiris warned.

"It will undermine the principle of sovereign equality. A report as preposterous as this will inflict grave damage to the UN system."

Peiris said he met Colombo-based diplomats Thursday to drum up support for Sri Lanka's stand against the UN report.

"We are appealing to peoples' sense of justice," Peiris said of his meeting with diplomats.

"We must be given space to deal with our problems."

According to parts of the report leaked at the weekend, the panel recommends an enquiry into 'credible allegations' of war crimes and crimes against humanity by both the government and the separatist Tamil Tiger rebels.

It also states that 'tens of thousands' of people died between January and May 2009 in the final government offensive that resulted in the defeat of the separatist Liberation Tigers of Tamil Eelam, ending a decades-old conflict.

The panel headed by former Indonesian attorney general Marzuki Darusman had transformed itself into an investigative body, Peiris said, insisting that the panel had no authority to probe any allegations against Sri Lanka.

The panel had also listed alleged violations by the rebel forces, saying they had intentionally used civilians as human shields.

Peiris said Sri Lanka was ready to engage the UN on taking the country's reconciliation process forward.

"It is the duty of the UN to reach out to Sri Lanka as we emerge from the darkest period of our history," the minister said.

Sri Lanka had previously managed to stave off censure at the UN Human Rights Council thanks to the support of two key allies, China and Russia.

Colombo vehemently argues that no civilians were killed during its offensive against the Tigers. But Sri Lanka has been fiercely criticised by international rights groups for failing to investigate deaths during the brutal fighting.

The US has led calls for an independent probe and warned Sri Lanka it risked an international commission of inquiry unless there was a credible domestic investigation.

President Mahinda Rajapakse has asked his supporters to turn this year's May Day rally into a demonstration against any UN war crimes investigation.