"Americans in the region should consider leaving, or be prepared to leave if the situation worsens," the embassy writes on its website.
American citizens are also advised to be prepared for disruptions to air traffic, with closed airspace and canceled flights.
The text does not refer to any specific reason, except that the region is unstable and that Iran has previously attacked American interests.
According to CBS News sources, the US and Israel are preparing major attacks on energy facilities in Iran, possibly as early as this weekend.
