More than 500 American citizens based in Bahrain are to be evacuated because of the threat of terrorism.
Security is tight in Bahrain, amid fears of anti-US violence
The Pentagon approved plans for the "temporary relocation" of family members and non-emergency personnel amongst the US military staff stationed there.
The tiny Gulf kingdom of Bahrain hosts about 5,000 Americans and the US Navy's Fifth Fleet.
The US claims it has "credible" news of planned attacks in Bahrain and earlier urged its citizens to consider leaving.
The evacuation order comes days after Bahrain released six men arrested on suspicion of links to al-Qaeda network.
The order was approved by Defence Secretary Donald Rumsfeld.
The 650-odd US citizens are likely to be moved back to America for a month, for the time being.
Security stepped up
"The Department of State warns US citizens to defer travel
to Bahrain. American citizens currently in Bahrain are urged to consider departing," the US embassy in the Bahraini capital, Manama, advised earlier on Friday.
"The Department has received information that extremists are planning attacks against US and other Western interests in the kingdom of Bahrain. Credible information indicates that extremists remain at large and are planning attacks in Bahrain."
The warning referred to recent attacks in Saudi Arabia, where foreign nationals, including Americans, have been killed and injured in a series of shootings and bombings.
US State Department deputy spokesman, Adam Ereli, said there were "indicators of people planning attacks " in the region, and additional information that Bahrain might be targeted.
Last week, Bahraini authorities released six people a day after they were arrested on suspicion of planning to commit "dangerous acts".
Bahrain has recently stepped up security on the bridge which links it to Saudi Arabia.
Many westerners relocated to Bahrain following the recent unrest in Saudi Arabia.