U.S. President Barack Obama says the threat that has shut down 19 embassies and consulates in the Middle East and Africa is "significant enough" that the United States is "taking every precaution."
The State Department has closed the diplomatic posts through Saturday and issued travel warnings amid intelligence that al-Qaida may be planning an attack.
Mr. Obama said in an interview broadcast late Tuesday that Americans should still take vacations, but do so with "some common sense and some caution."
He said terrorists depend on people feeling terrorized, and that Americans are "going to live our lives."
One of the State Department's precautions was ordering the evacuation of non-essential staff from Yemen because of what it calls the "extremely high" potential of a terrorist attack.
It has issued a travel warning for Yemen and urges U.S. citizens who decide not to leave to limit their movements.
Britain has also evacuated all staff from its embassy in Yemen.
U.S. media reports say intelligence agencies intercepted communications between al-Qaida leader Ayman al-Zawahri in Pakistan and the head of al-Qaida's branch in Yemen, Nasir al-Wuhayshi. The reports say al-Zawahri ordered the Yemen branch to carry out an attack as early as this past Sunday. But U.S. officials were unable to pinpoint the exact time and place of the possible attack.
Analysts say the communications indicate al-Zawahri is working through the affiliates because they say the al-Qaida core has been substantially weakened.
The U.S. diplomatic posts to stay closed all week are Amman, Cairo, Sanaa, Abu Dhabi, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Antanarivo, Bujumbura, Djibouti, Khartoum, Kigali, and Port Louis.