Amnesty International called for an independent probe on Saturday 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.
The report followed weeks of allegations that large numbers of civilians had been killed as the army closed in on Tamil Tiger rebels to end the decades- long war.
Amnesty's Asia Pacific director Sam Zarifi accused both sides of war crimes and called for an independent international probe.
"The Times report underscores the need for this investigation and the UN should do everything it can to determine the truth about the bloodbath' that occurred in northeast Sri Lanka," Zarifi said in statement.
The statement said the UN "must immediately publicise its estimate of the number of civilians killed by the two sides in the final weeks of fighting".
Sri Lanka's Human Rights Minister Mahinda Samarasinghe dismissed Amnesty's call and said the organisation was being "ridiculous to keep harping on things they cannot substantiate," he told AFP by telephone from Geneva.
The Colombo-based 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," defence ministry spokesman Lakshman Hulugalle said. "We totally deny the allegation that 20,000 people were killed."
Amnesty said, however, that it continues 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.
"The UN must address the war crimes and grave human rights violations that have occurred -- and could still be occurring -- in Sri Lanka," Zarifi said.
He said that despite repeated calls, the Sri Lankan government continued to restrict access to the camps by international humanitarian organisations, including the UN and the Red Cross.
"I am appealing to all these rights groups to let us get on with the job of resettling these people in their homes in the quickest possible time," Samarasinghe said.
The island's military claimed complete victory over separatist Tamil Tigers after wiping out the guerrillas' leadership nearly two weeks ago.