The top aide to the United Nations Secretary-General was told more than a week ago that at least 20,000 Tamil civilians were killed in the Sri Lankan government's final offensive against the Tamil Tiger rebels this month, The Times can reveal.

U.N. officials told Vijar Nambiar, Ban Ki-moon's chief of staff, that their figures indicated a likely final death toll of more than 20,000, during a briefing in preparation for Mr. Ban's visit to the region on May 23.

Two staff present at the meeting confirmed the exchange to The Times but Mr. Ban never mentioned the death toll during his tour of the battleground, which he described as the "most appalling scene" he had witnessed in his long international career.

The casualty figure, revealed by The Times Friday, triggered an international furor, with the Sri Lankan authorities denying the report and human-rights groups demanding an investigation into possible war crimes.

Lakshman Hulugalle, a Sri Lankan defense ministry spokesman, said: "These figures are way out . . What we think is that these images are also fake. We totally deny the allegation that 20,000 people were killed."

But, internationally, calls have been growing for an independent war-crimes investigations on both sides and for access by humanitarian groups to the war zone and the 270,000 Tamil civilians who are still being detained.

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