May 12 (Bloomberg) -- The civilian death toll in Sri Lanka is “unacceptably high,” the U.S. said, joining the United Nations and Britain in demanding an end to the conflict after hundreds of mostly ethnic Tamils were killed at the weekend.

The rebel Liberation Tigers of Tamil Eelam must disarm and allow civilians to leave the combat zone, the State Department said, while calling on the government to abide by its pledge to end “major combat operations and the use of heavy weapons.”

Ambassador Susan Rice met with French and British officials at the UN in New York yesterday to discuss the fighting, according to a statement.

The Tamil Tigers are holding out in a sliver of land near the northeastern port of Mullaitivu after being driven from their bases by government troops. The UN said yesterday the conflict has become a “bloodbath” as hundreds of civilians were killed in the government-declared no-fire zone where the rebels are sheltering.

UN Secretary General Ban Ki-moon is “appalled at the killing” and “deeply concerned by the continued use of heavy weapons,” his office said in a statement.

U.K. Foreign Secretary David Miliband, who last month visited Sri Lanka to call for a cease-fire, said civilians trapped in the no-fire zone are the “victims of what at the moment is a war without witness.”

‘Killing Must Stop’

“The killing must stop,” he told reporters at the UN in New York yesterday.

Sri Lanka’s government denies reports that at least 1,200 Tamil civilians have been killed in artillery shelling and accuses the Tamil Tigers of mounting a propaganda campaign to win international sympathy.

The group, which has been fighting for 26 years for a separate homeland in Sri Lanka, is designated a terrorist organization by the U.S., the European Union and India.

LTTE leaders are using civilians as human shields and have shot people trying to flee the no-fire zone, which is an area of less than 6 square kilometers (2.3 square miles), according to the Defense Ministry.

The government says troops have rescued more than 116,000 civilians from the zone since April 20 and the offensive is aimed at freeing even more.

Outside observers are barred from the conflict zone and accounts from the region are difficult to verify.

To contact the reporter on this story: Ed Johnson in Sydney at ejohnson28@bloomberg.net.

Last Updated: May 11, 2009 20:27 EDT