AFP - UN chief Ban Ki-moon on Monday categorically rejected allegations the world body had deliberately underestimated the death toll in the ethnic conflict in Sri Lanka.

"I categorically reject -- repeat categorically -- any suggestion that the United Nations has deliberately underestimated any figures," he told the UN General Assembly.

He was reacting to press reports citing confidential UN reports that more than 20,000 civilians were killed by Sri Lankan army shelling.

"In regard to some reports in the media, I should emphasize that the final total is not yet known," the UN secretary general said. "Most of these figures do not emanate from the UN and most are not consistent with the information at our disposal."

Ban however made it clear that "whatever the total, the casualties in the conflict were unacceptably high -- as I have also said repeatedly."

Saturday, the London-based Amnesty International called for an independent probe into the number of civilians killed in the final weeks of Sri Lanka's civil war and also urged the UN to reveal its own estimates.

The call by the rights group followed a report in the Times of London newspaper on Friday citing confidential UN reports that more than 20,000 civilians were killed by Sri Lankan army shelling.

Commenting on the report, UN spokeswoman Michele Montas told a press briefing Monday: "The number of 20,000 is not a UN number."

She said she checked with UN officials in Colombo earlier in the day and the response was that "they have absolutely no idea where that number came from."

The Times of London report followed weeks of allegations that large numbers of civilians had been killed as the Sri Lankan army closed in on Tamil Tiger rebels and finally crushed the 30-year-old insurgency by the separatist Liberation Tigers of Tamil Eelam (LTTE) last month.

Amnesty's Asia Pacific director Sam Zarifi accused both sides of war crimes and called for an independent international probe.

The Colombo government, which has rejected demands by the UN Human Rights Council for a fact-finding mission on the war crimes allegations, on Friday angrily dismissed the Times report.

"These figures are way out," Sri Lankan defense ministry spokesman Lakshman Hulugalle said. "We totally deny the allegation that 20,000 people were killed."

Amnesty said, however, that it continued to receive reports of widespread human rights violations, with more than 280,000 people displaced by the recent fighting and now restricted to state-run welfare camps in the island's north.