It said the warning was issued "to remind U.S. citizens of the need to remain vigilant due to a heightened threat of terrorist actions that may target civilians, including the possibility of attacks by nonconventional weapons.

"While conventional weapons such as explosive devices pose a more immediate threat in many areas overseas, terrorist use of nonconventional weapons, including chemical or biological agents, must be considered a growing threat," it added.

The State Department did not cite any specific reason why it had decided to issue the warning, its first since Nov. 20. The last warning was issued after the broadcast of an audiotape believed recorded by al Qaeda chief Osama bin Laden.

At the U.N. Security Council on Wednesday, Powell used satellite photographs and audiotape of intercepted Iraqi conversations to argue that Iraq had hidden illegal arms by bulldozing chemical weapons sites, hiding rocket launchers under palm trees and mounting mobile bioweapon labs on trucks.

He also spoke of the possibility that Iraq could give such weapons to "terrorist" groups and of links between Baghdad and al Qaeda. He did not, however, suggest Iraq was connected to Sept. 11 or that it exercized any control over al Qaeda.

"U.S. citizens and interests are at a heightened risk of terrorist attacks, including by groups with links to al Qaeda," the State Department said in its warning, saying these might include "suicide operations, assassinations or kidnappings."

"These individuals and groups have proved that they do not distinguish between official and civilian targets. Because security and security awareness have been elevated within the United States, terrorists may target U.S. interests overseas," it added. "Private Americans should remain vigilant with regard to their personal security and exercise caution."

The statement repeated previous U.S. warnings that attacks were possible on places of worship and schools as well as other places where Americans or foreigners are known to congregate or visit, such as residential areas, clubs, restaurants, hotels, outdoor recreation events or resorts and beaches.

Among other things, the department encouraged U.S. citizens living abroad to keep key documents like passports up to date and easily accessible and to keep supplies of food, water and medicine if they lived in an area where political unrest, street demonstrations or other disruptions were common.