Two warships have already left Liberia waters, and a third is due to leave mid-week, defence officials said.

The UN is due to start deploying its largest force in the world - some 15,000 troops - on Wednesday.

About 100 US soldiers remain on the ground in Liberia, mainly stationed around the US embassy.

The US forces were sent to Liberia in August to support Nigerian-led peacekeepers who helped restore calm to the capital, Monrovia, after months of fighting between government troops and rebels.

However, the peacekeepers have not deployed to the rest of the country, where tension between the rival forces remains high.

Stretched by military commitments in Iraq and Afghanistan, President George W Bush said the American troops would be withdrawn by 1 October.

The US warships carried more than 4,000 sailors and Marines, but most of them never set foot on the mainland.

Two of the ships - the USS Carter Hall and the USS Nashville - left Liberian waters over the weekend.

The third vessel - the USS Iwo Jima - is expected to follow in the next few days.

"Since our mission was to facilitate Ecomil [West African peacekeeping forces] efforts to stabilise Monrovia and create conditions for humanitarian relief efforts to resume, that mission has been largely accomplished," said Pentagon spokesman Lieutenant Dan Hetlage.

The 3,500-strong Ecomil force will swap their green helmets for the blue of the UN peace force, Unmil, on Wednesday.

More than a thousand Liberians have fled to neighbouring Guinea amid threats of attacks by rebels in central Liberia and reports of summary executions and rape of civilians, the United Nations refugee agency said Tuesday.

A spokesman for the UN High Commissioner for Refugees (UNHCR), Peter Kessler, said some 1300 Liberian refugees had been registered so far this week in the Guinean border areas of Bignamou and Baala.

"They said they had left their homes in Lofa county following threats of rebel attacks," Kessler told journalists.

"Some of these departures have been fed by reports that rebels have been harrassing civilians, looting and raping women. There have also been unconfirmed reports of summary executions," Kessler said.

Fighting between rebels of the Liberians United for Reconciliation and Democracy (LURD) and government forces in nearby Bong and Nimba countries forced more than 6,000 Liberians to flee to Guinea over the past two weeks, he added.