LONDON - More than 20,000 Tamil civilians were killed in the final days of Sri Lanka's military operation to defeat Tamil Tigers rebels, The Times newspaper reported on Friday.

Sri Lanka's authorities say their forces stopped using heavy weapons on April 27 in a no-fire zone where an estimated 100,000 Tamil civilians were sheltered and blame civilian casualties on rebels hiding among the civilians, the paper said.

Citing confidential U.N. documents it acquired, The Times said the civilian death toll in the no-fire zone soared from late April, with around 1,000 civilians killed daily until May 19. That was the day after Vellupillai Prabhakaran, leader of the Liberation Tigers of Tamil Eelam (LTTE), was killed.

The final civilian death toll could be more than 20,000, said the paper.

U.N. High Commissioner of Human Rights Navi Pillay has said the LTTE recruited child soldiers and used civilians as human shields during the conflict, while the military had indiscriminately shelled areas packed with civilians.

Both sides have denied the allegations.

Sri Lanka has called a Western-led push for a rights and war crimes probe hypocrisy and a violation of its right to destroy the LTTE, which is listed as a terrorist organization by more than 30 countries.

The United Nations estimates that between 80,000 and 100,000 people died in what was one of Asia's longest modern wars, erupting in earnest in 1983 when the Tigers began to fight for a separate state for Sri Lanka's minority Tamils.

(Writing by Jon Boyle; Editing by Robert Woodward)