The first helicopter to return from Monrovia arrived at Lungi airport on the outskirts of Sierra Leone's capital Freetown with 24 American civilians after nightfall, Commander Bob Anderson, a US military official said.
The evacuation involving five helicopters would continue throughout the night, he added.
He said 420 US nationals and dependants were in Monrovia. Another 200 foreign nationals - including about 30 Britons - who have taken refuge in the US Embassy compound in the city would be considered for evacuation on a case-by-case basis.
"So far as we know, no Britons are leaving at this stage," said a Foreign Office spokesman in London.
The decision to begin the evacuation came after ferocious fighting forced thousands to flee their homes. Rebel fighters were holding hundreds of civilians and West African peacekeepers captive.
The UN began flying out some of its staff and Oxfam announced that it had successfully evacuated six of its staff from the southern town of Greenville.
Aid workers described how they had been forced to lie low as gunfire, artillery and mortar shells echoed all around them amid fears of a return to all-out war in an on-off conflict which has killed 150,000 since 1989.
Oxfam said a further six staff were sheltering in the Oxfam house some 200 yards from the US Embassy. Save the Children said it had seven aid workers in hiding.
Fighters loyal to the fugitive warlord Roosevelt Johnson held several hundred Lebanese and Liberian civilians and about 20 Nigerian peace-keepers hostage in a barracks where Mr Johnson was reported at one stage to be hiding.
The fighting and looting began at the weekend with a stand-off between supporters of Mr Johnson and soldiers loyal to the ruling council of state, which sacked him as its rural development minister and ordered his arrest on charges of murder during a clash with militia rivals.
The six-member council, which was set up under the latest of a long line of peace deals, comprises the main faction leaders as well as civilians.
Shooting began again yesterday near the army barracks.
Diplomats, officials and witnesses said that about 40 Lebanese, mostly women and children, about 20 Nigeria peace-keepers and several hundred Liberians were being held in the barracks.
The barracks of the Armed Forces of Liberia, which are now just another militia faction, has been the focus of fighting which has flared sporadically elsewhere in the city.
Sustained bursts of shooting have mingled over the past few days with mortar, artillery and rocket-propelled grenade blasts.
There was no reliable casualty toll from the fighting. Medical aid workers spoke of at least six dead and 40 wounded.
West African peace-keepers of the Ecomog force patrolled the city. A spokesman said that they were trying to broker a ceasefire.
"At the end of the day, all of us have to come to the negotiating table. It is my appeal to all of the parties to cease fire,"
Major-General John Inien-ger said, offering his Ecomog force as mediators.
Ghana, which is current chairman of the Economic Community of West African States, which sent in the peacekeepers, told Mr Johnson that they would ensure his security during talks on his differences with the council of state. However, his whereabouts are not known. Last August's peace deal envisaged a ceasefire, disarmament and elections taking place within a year.
But skirmishing militiamen have repeatedly breached the ceasefire, while the process of disarmament has yet to start.
Liberia was ruled by descendants of the original slaves until Master Sgt Samuel Doe, a member of the indigenous Krahn people, seized power in a bloody 1980 coup and ruled brutally with blatant ethnic bias.
Roosevelt Taylor, a US-educated former Doe cohort who fell out of favour, launched a rebellion from Ivory Coast after escaping from a Massachusetts jail where he was awaiting extradition to Liberia on charges of embezzling from the Doe government.
Taylor's rebellion splintered into gruesome ethnic warfare marked by hideous massacres.
Doe himself was tied to a chair by a splinter rebel group and tortured to death; at one point his captors tried to make him eat his severed ear.Reuse content