The notice said that those "travelling or living abroad" should be especially vigilant, as intelligence indicated that the planned attacks were "specifically targeting American citizens". The Foreign Office in London said it was consulting the US about the warning.
The notice did not name specific terrorist groups or locations, but officials noted separately that the group headed by Osama bin Laden was seen as a prime threat. Bin Laden, whose headquarters are thought to be in Afghanistan, is the chief suspect in the bombings of US embassies in Kenya and Tanzania last year. Although he is at the top of the US's "most wanted" list and the US has put pressure on Taliban leaders to give him up, he has evaded capture.
An unnamed government source was quoted by the Los Angeles Times as saying that there had been "a slew of threat reports" in recent weeks, mainly relating to Mr Bin Laden, but "one emerged in the past couple of days that seemed much more credible and specific".
The Secretary of State, Madeleine Albright, reinforced the warning on television yesterday, but said there were "a variety of groups that we have been concerned about and are watching".
The White House National Security Adviser, Sandy Berger, said on another TV interview that the information was credible. "Americans are among the targets," he said. "It's not specific with respect to location... This is not just the Middle East, but any location."
Among measures Americans are advised to take is to beware of appearing in big groups, to keep a low profile and to vary daily routines.
Travel advisory notices are routinely issued by the US State Department, but usually apply to named countries following particular US military or other action. But "global" warnings have become more common: this is the fifth since October. The last came after the UN approved sanctions against Afghanistan in connection with Mr Bin Laden.Reuse content