A US Special Forces commando unit and 18 soldiers arrived in Monrovia yesterday to prepare the evacuation of civilians, the Pentagon said.
UN sources in the Liberian capital said last night the evacuation of US nationals and other foreigners was already under way.
Fighters loyal to fugitive warlord, Mr Roosevelt Johnson, held several hundred Lebanese and Liberian civilians and about 20 Nigerian peacekeepers hostage in a city centre barracks where Mr Johnson was reported at one stage to be hiding.
The fighting and looting began on Saturday with a stand off be tween supporters of Mr Johnson and the ruling council of state, which sacked him as rural development minister and ordered his arrest for murder following a clash with militia rivals.
The six member council, set up under the latest of a long line of peace deals groups main tact ion leaders and civilians.
In New York, the UN Security Council was preparing a statement on the fighting in Liberia.
The 15 member council was expected to issue a formal statement expressing grave concern about the sudden upsurge of fighting in Monrovia and the deterioration of the situation in the west African country as a whole.
The council wants an effective ceasefire in Liberia and the release of all hostages held by Liberian gunmen in Monrovia.
A UN official said three helicopters provided by the US to the African peacekeeping force had been attacked and rendered "not usable" at Monrovia airport during the fighting in the last four days. The attack was blamed on the ULIMO-J faction.
Some 220 UN personnel are in Monrovia, including 135 civilian and military members of the UN Observer Mission in Liberia (Unomil). The US has 450 nationals in Liberia. Thousands of Liberians have sought refuge from the fighting at the US compound at Mamba Point.
The US Defence Secretary. Mr William Perry, said Washington was sending additional security forces to Liberia, but did not provide figures.
The British ambassador to the UN, Sir John Weston, said yesterday the latest situation in Liberia meant that "the credibility of the Security Council is at stake".
He noted that the council had previously warned the Liberian factions that the international community's patience was running out, and that it might be time to "take a step back" and reassess UN support for the continually violated peace process in Liberia. Unomil's current mandate runs out on May 31st.
The Security Council is expected to discuss today a report on Liberia by the UN Secretary General, Dr Boutros Boutros Ghali, in which he warns of a possible "major humanitarian disaster" unless additional resources were provided for the African peacekeeping force.