WASHINGTON?- The US State Department on Tuesday ordered non-essential personnel at the US 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 US citizens in Yemen should leave immediately because of an "extremely high" security threat level.

"As staff levels at the Embassy are restricted, our ability to assist US citizens in an emergency and provide routine consular services remains limited," the travel warning said.

A US intelligence official and a Mideast diplomat?said 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 US has hit terrorist leaders in Yemen with drone strikes. On Tuesday, Yemeni security officials said a suspected US 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 The office said the British embassy staff were "temporarily withdrawn to the U.K." on Tuesday, but declined further comment.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 UK had said the embassy would be closed until the end of the Muslim festival of Eid later this week.

AQAP, gathered in small cells scattered across Yemen's vast under-governed regions, has proven to be a tenacious enemy.

Officials say al-Zawahri, who took over for Osama bin Laden and works from Pakistan, has reached out to the Yemeni branch, further signaling the AQAP is once again looking to target US and Western interests after a sustained period of more local and regional focus.

The latest warnings raise questions about how successful America's war on terror has been and whether the terror group has been able to reorganize and reconstitute itself since Osama bin Laden's death in May 2011 in Pakistan.

And, although US officials agreed a year ago to restart one-frozen military aid to Yemen, it's unclear how much of the new aircraft and weapons have arrived. The US military is once again training Yemeni special operations forces and has delivered more than a dozen helicopters to the Yemeni military, US defense officials said.

The embassy closures came one day after a meeting between President Barack Obama and Yemeni President Abdo Rabu Mansour Hadi.

Even though AQAP lost US citizen Anwar al-Awlaki - one of its key inspirational leaders - to a US drone strike in 2011, al-Wahishi and the group's master bomb maker, Ibrahim Hassan al-Asiri, remain on the loose and determined to target the US and other Western interests.

The group is linked to the attempted Christmas Day 2009 bombing of an airliner bound for Detroit and explosives-laden parcels intercepted aboard cargo flights a year later - both incidents involving al-Asiri's expertise.

In recent years, however, AQAP has been focused more on making gains at home, taking advantage of an unstable government and overstretched military.