WASHINGTON (AP) --The U.S. State Department on Tuesday ordered non-essential personnel at the U.S. Embassy in Yemen to leave the country following the threat by al-Qaida that has caused temporary shutdowns of 19 American diplomatic posts across the Middle East and Africa.

The department said in a travel warning that it had ordered the evacuation ``due to the continued potential for terrorist attacks'' and said U.S. citizens in Yemen should leave immediately because of an ``extremely high'' security threat level.

Defense Department Press Secretary George Little said the U.S. Air Force transported State Department personnel out of Sanaa early Tuesday. ``The U.S. Department of Defense continues to have personnel on the ground in Yemen to support the U.S. State Department and monitor the security situation,'' Little said.

A U.S. intelligence official and a Mideast diplomat told The Associated Press that the current shutdown of embassies in the Middle East and Africa was caused by an intercepted secret message between al-Qaida chief Ayman al-Zawahri and Nasser al-Wahishi, the leader of the Yemen-based al-Qaida in the Arabian Peninsula, about plans for a major terror attack. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

AQAP has been widely considered al-Qaida's most dangerous affiliate.

The U.S. has hit terrorist leaders in Yemen with drone strikes. On Tuesday, Yemeni security officials said a suspected U.S. drone killed four alleged al-Qaida members in a volatile eastern province. The drone fired a missile at a car carrying the four men, setting it on fire and killing all of them, the officials said.

The Yemeni officials, speaking on condition of anonymity because they are not allowed to talk to the media, said they believe one of the dead is Saleh Jouti, a senior al-Qaida member. It's the fourth drone attack in the past week to hit a car believed to be carrying al-Qaida members.

The State Department on Sunday closed a total of 19 diplomatic posts until next Saturday. They include posts in Bangladesh and across North Africa and the Middle East as well as East Africa, including Madagascar, Burundi, Rwanda and Mauritius.

Britain's Foreign Office also announced that it had evacuated all staff from its embassy in Yemen due to security concerns. The office said the British embassy staff were ``temporarily withdrawn to the U.K.'' on Tuesday, but declined further comment. Previously, the U.K. had said the embassy would be closed until the end of the Muslim festival of Eid later this week.