"It remains our intention to publish the report of the panel of experts on Sri Lanka in full and without amendment," deputy UN spokesman Farhan Haq told a press briefing.
He added that talks were being held with Sri Lankan authorities on an offer to add their comments to the report into the alleged deaths of tens of thousands of people when government forces launched a final offensive against Tamil separatists in 2009.
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Sri Lanka's Foreign Minister G.L. Peiris earlier asked UN Secretary General Ban Ki-moon not to release the report which he called "preposterous".
"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.
The United Nations said the report would be released as soon as possible but gave no day and did not indicate whether there was a deadline.
"Discussions are continuing today. We have tried to see whether they are willing to avail themselves of the idea of publishing their own response to the report," said Haq, adding: "That opportunity is not an indefinite one."
"We are continuing in good faith to offer them their right to have their response added," he said.
According to parts of the report leaked to the Sri Lankan media at the weekend, the UN panel recommends an inquiry into "credible allegations" of war crimes and crimes against humanity by both the government and the separatist Tamil Tiger rebels.
It 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 Sri Lankan government argues that no civilians were killed during its offensive against the Tigers. President Mahinda Rajapakse has called on supporters to turn this year's May Day rally into a demonstration against any UN war crimes investigation.
The UN leader set up the panel last year amid growing frustration at the Sri Lankan government's accounting for the military offensive.
He said the United Nations considered the panel carried out its work "conscientiously", rejecting Sri Lankan accusations of bias.
