The Foreign Office broadcast appeals via BBC World Service to about 50 expatriates who remain in the capital, Freetown.

Nigeria is sending extra troops to defend the capital as rebel troops rampage through the streets.

The move follows the West African coalition force's failure to halt the rebel onslaught.

The rebels forced their way into the city on Tuesday and set fire to a main government office building and the city's police station.

Terrified residents huddled indoors as the government warned that anyone seen in the streets would be regarded as a rebel and shot.

The Foreign Office told all Britons to leave the country, as the rebel Revolutionary United Front launched a fresh offensive against the government and the Nigerian-led Ecomog coalition.

Soldiers touring Freetown

An FO spokesman said: "We have been broadcasting messages on the World Service there, telling them to leave immediately or keep their heads down."

The High Commission in Freetown is temporarily closed because of the fighting.

But officials in London are in touch with people in the city and said there were no reports of Britons being injured.

The United Nations' entire military observer mission to Sierra Leone was at the airport on Wednesday awaiting evacuation to the Guinean capital Conakry.

The BBC's Mark Doyle: "There has been shooting throughout the morning"

BBC correspondent Mark Doyle said that the airport was surrounded by Ecomog soldiers, but UN staff did not know who was controlling the city.

The troubled West African state embroiled the Foreign Office in scandal after it emerged that British mercenaries were aiding ousted President Ahmed Tejan Kabbah, using British arms supplied in breach of a United Nations embargo.

The mercenaries claimed British diplomats were aware of their plans and the "Sandline affair" - named after the mercenaries' company - saw ministers up to and including Foreign Secretary Robin Cook facing repeated parliamentary questioning over what they did and did not know.