The State Department said it was extremely dangerous to travel in Sierra Leone, because of possible attacks by rebel forces.

The UK government has also advised an estimated 200 of its nationals in Sierra Leone to leave.

The US move comes after the government of Sierra Leone accused neighbouring Liberia of being behind the current rebel offensive which earlier this week came within about 30km of the capital, Freetown.

Sierra Leone's Information Minister, Julius Spencer, said a number of senior Liberian officials were actively involved.

The rebels, loyal to the ousted military government, say they have mounted an offensive to force the authorities to negotiate.

They also want the release of their leader, Foday Sankoh. He is being held in Freetown where he has been sentenced to death for treason.

The BBC West Africa Correspondent, Mark Doyle, says it has been known for some time that a number of Liberians are fighting with the rebels but clear evidence of official Liberian Government involvement has not been available.

A key area of Sierra Leone held by the rebels is on the border with Liberia.

Fighting for peace

The Sierra Leone rebels say they are fighting to try and force the government into peace talks.

But the government says that it will only talk if the rebels lay down their arms.

Our correspondent says a key problem is that government politicians in Freetown do not want to be seen to be making compromises with the rebels who are unpopular in the capital and in other towns because of the atrocities they have committed against civilians.

Earlier, the United Nations Security Council expressed concern about an increase in rebel attacks and called on all countries to observe an arms embargo against the rebels and Liberia.

The UN has a small military observer mission in Sierra Leone which has evacuated non-essential personnel out of the country but has not closed down its operations.