The embassy has only been open a few months

A senior US defence official - speaking on condition of anonymity - told the BBC that "specific information" about a threat against a "specific target" had been received.

The warning indicated the target, the group and the method of attack, US officials say.

Sources in Kenya say the threat is against the embassy building itself.

Hand of al-Qaeda

All US interests are on the highest state of alert.

A State Department spokesman said the mission in the capital Nairobi was likely to remain closed until Tuesday.

The new embassy building on the edge of the city has only been open a few months.

Five years ago, the US embassy was destroyed by a car bomb, which has been blamed on al-Qaeda.

Tourist warning

And in November last year, an Israeli hotel on the Kenyan coast was bombed and a charter plane narrowly missed being hit by a surface-to-air missile on take-off from the second city Mombasa.

For the past month, the US and Britain have warned tourists not to visit Kenya because of fears of a new al-Qaeda strike.

British Airways has suspended all flights to the country.