The US military has evacuated its non-essential Government personnel from Yemen due to the high threat of attack by al Qaeda which has caused temporary shutdowns of 19 American diplomatic posts across the Middle East and Africa.

The State Department said it 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.

Defence Department press ­secretary George Little said the US Air Force transported State Department personnel out of Sanaa early yesterday, adding: "The US Department of Defence continues to have personnel on the ground in Yemen to support the US State Department and monitor the security situation."

A US intelligence official and a Middle East diplomat said the current shutdown of embassies in the Middle East and Africa was caused by an intercepted secret message between al Qaeda chief Ayman al Zawahri and Nasser al Wahishi, the leader of the Yemen-based al Qaeda in the Arabian Peninsula (Aqap), about plans for a major terror attack.

Aqap has been widely considered al Qaeda's most dangerous affiliate.

The alert came as Yemeni ­security officials said a suspected US drone killed four alleged al Qaeda members in a volatile eastern province. The drone fired a missile at a car carrying four men, setting it on fire and killing all of them, the officials said.

The Yemeni officials said they believe one of the dead was Saleh Jouti, a senior al Qaeda member. It was the fourth drone attack in the past week to hit a car believed to be carrying al Qaeda members.

The State Department on Sunday closed 19 diplomatic posts for a week. 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 it had evacuated all staff from its embassy in Yemen due to security concerns. The office said the staff were temporarily withdrawn to the UK, but declined further comment.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 from Osama bin Laden and works from Pakistan, has reached out to the Yemeni branch, further signalling 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 reorganise and reconstitute itself since bin Laden's death in May 2011 in Pakistan.

Although US officials agreed a year ago to restart military aid to Yemen, it is unclear how much of the new aircraft and weapons have arrived. The US military is again training Yemeni special operations forces and has delivered more than a dozen helicopters to the military, US defence officials said.

The embassy closures came a 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 free 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.