Schoolchildren in Sri Lanka have been signing petitions against the public release of the UN's report [Reuters]

The Sri Lankan government has warned the United Nations that publicly releasing a report on alleged war crimes in the country's civil war could harm post-war ethnic reconciliation efforts.

Gamimi Peiris, the country's external affairs minister, said in Colombo on Thursday that the UN panel had overstepped its mandate.

Rather than acting only as an advisory body to Ban Ki-moon, the UN secretary-general, it had taken on an investigative role, Peiris said.

"The publication of this report will cause irreparable damage to the reconciliation efforts of Sri Lanka. It will damage the UN system too. This UN report is preposterous."

The report, which was handed to Ban last week, harshly criticised both the Sri Lankan government and the Liberation Tigers of Tamil Eelam (LTTE) rebels on their conduct during the civil war, saying that there were "credible allegations" of war crimes and crimes against humanity against both sides.

While the UN has not released the report officially, media reports have described parts of it. Peiris was given a copy of the report nine days ago.

He stressed that the Sri Lankan government had never formally accepted Ban's appointment of the panel, and warned the UN secretary-general against initiating a full-scale inquiry into suspecting civilian deaths in the final days of fighting.

The report lists violations by both sides in the conflict, saying that "tens of thousands" of people died between January and May 2009 in a final government offensive that resulted in the defeat of the LTTE.

The government argues that no civilians were killed in that offensive.

Peiris said his country was ready to engage with the UN in a constructive discussion on taking the reconciliation process forward.

Mahinda Rajapakse, the Sri Lankan president, had earlier asked his supporters to turn this year's May Day rally on May 1 into a demonstration against any UN war crimes investigation.