Two twin-rotor US helicopters flew in from from the direction of the ships towards the US embassy in Monrovia's Mamba Point diplomatic area.

But a US military source said the helicopters was flying in supplies to the embassy and there was "no troop deployment" for the moment.

A US defence official in Washington said that three amphibious warships loaded with marines were meant as a "powerful message" to Liberia's warring factions.

The official, who asked not to be identified, said there were no plans to send troops ashore but the US warships were positioned close by in case their assistance became necessary.

"People in Monrovia can see the ships right off the shore, I would think that would be a pretty powerful message," the official said.

The United States earlier this month despatched three ships -- the USS Iwo Jima, Carter Hall and Nashville -- off the coast of Monrovia with some 3,000 marines on board.

Meanwhile, US military officials on Monday travelled with the ECOMIL west African peacekeeping force to the rebel-held port area of Monrovia, a key gateway for food and humanitarian supplies, to negotiate its re-opening with the insurgents.

US sources said the west African force hoped to sign an agreement with rebels from the Liberians United for Reconciliation and Democracy (LURD) group on Tuesday allowing peacekeepers to secure the area.

Under the agreement, the LURD is due to open the harbour to peacekeepers and aid workers and allow in humanitarian supplies and food, and then pull back to the Po River, about 20 kilometres (12 miles) from Monrovia.

The port is crucial for resolving the humanitarian crisis in Monrovia, where some 250,000 displaced people are suffering the effects of an increasingly acute shortage of food, water and medical supplies.

The LURD has besieged Monrovia for over two months and had vowed not to pull back from its positions until Taylor left the country.

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