"Americans should keep a low profile, vary routes and times for all required travel, and treat mail from unfamiliar sources with caution," the US State Department said.

Osama bin Laden has been indicted for the attacks

The department said it was not issuing the "worldwide caution" in response to a specific threat or event, but noted that Saudi millionaire Osama bin Laden, accused by Washington of masterminding the bombings, continued to threaten Americans.

It said terrorists might take actions against Americans and American interests to mark the blasts on 7 August last year at the embassies in Kenya and Tanzania.

It also noted the coming anniversary of the retaliatory strikes by the US on 20 August against targets in Afghanistan and a factory in Sudan, which Washington said were involved in terrorism.

At least nine people have been charged by US courts with involvement in the bombings; two are facing extradition from the UK.

Civilian targets

The State Department said it continued to receive "reports that prompt concern about the safety and security of both official US Government personnel and private American citizens".

Nairobi has opened an exhibition marking the bombing

Mr bin Laden and others, "continue their threats against the United States and have not distinguished between military and civilian targets", it added.

Reports say government analysts estimate 2,400 threats or incidents have been aimed at US interests overseas since the bombings - more than double the same period a year ago.

The State Department has spent $1.5bn rebuilding and enhancing security at posts around the world in the last year.

Secretary of State Madeleine Albright is to host ceremonies marking the first anniversary of the bombings in the State Department building in Washington.