WASHINGTON: The United States on Tuesday ordered Americans to leave Yemen “immediately” amid a region-wide alert linked to electronic intercepts from al Qaeda leader Ayman al-Zawahiri to the group's Arabian Peninsula franchise.

The alert came hours after a drone strike killed four suspected al Qaeda militants in Yemen and two days after the closure of some two dozen embassies in the Middle East and Africa.

The State Department said it had pulled all non-essential personnel from Yemen, and the Pentagon said the US Air Force had flown staffers out early Tuesday.

“We are concerned about a threat stream indicating the potential for terrorist attacks against US persons or facilities overseas, especially emanating from the Arabian Peninsula,” State Department spokeswoman Jen Psaki said in a statement.

“As such, the department is taking appropriate steps to protect our employees, including local employees and visitors to our facilities.”

Pentagon spokesman George Little said defense department personnel remain “on the ground in Yemen to support the US State Department and monitor the security situation.”

Britain meanwhile announced the temporary withdrawal of all personnel from its embassy in Yemen, saying it would remain closed “until staff are able to return.”

Intercepts between Zawahiri and Nasser al-Wuhayshi, the leader of al Qaeda’s Yemen affiliate, sparked the closure of the US missions overseas and a worldwide travel alert, US media reported.

The New York Times said Monday that the electronic communications last week revealed that Zawahiri had ordered al Qaeda in the Arabian Peninsula to carry out an attack as early as last Sunday.

CNN meanwhile reported that Zawahiri told Wuhayshi to “do something,” sparking fears in Washington and Sanaa.

As a result, roughly two dozen US diplomatic posts were shuttered across the Middle East Sunday, and the State Department said 19 would remain shut through Saturday.

Al-Qaeda in the Arabian Peninsula (AQAP) is seen as the terror network’s most capable franchise following the decimation of its core leadership in Afghanistan and Pakistan in recent years.

The Yemen-based group has attempted a number of attacks on US soil, including a bid to bring down a passenger plane in 2009 by a man wearing explosives in his underwear and a failed plot to send bombs concealed in printers.

The United States in turn has launched scores of drone strikes in Yemen, where the militant group thrives in vast, lawless areas largely outside the government's control.A drone strike in Yemen Tuesday struck a vehicle, killing four suspected al Qaeda militants “in a ball of fire,” a tribal source told AFP.

One of the four was on a list released by Yemeni authorities of 25 al Qaeda operatives suspected of plotting attacks to coincide with the end of the Muslim holy month of Ramazan later this week, according to the source.

It was not immediately clear if the State Department alert was related to the drone strike. US officials, who rarely acknowledge the covert drone program, could not be reached for comment.

Several US allies, including Britain, France, Germany and Norway, have also announced closures of some of their missions in the region.

Lawmakers in Washington described the threat level as very serious, with some invoking the September 11, 2001 attacks.

Michael McCaul, chairman of the House Homeland Security Committee, dubbed the intelligence “probably one of the most specific and credible threats I've seen, perhaps, since 9/11.”

Saxby Chambliss, vice chairman of the Senate Intelligence Committee, said the level of chatter among alleged terrorists was “very reminiscent of what we saw pre-9/11”.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, told ABC News the threats were “more specific” than previous ones, although the exact target was unknown.ABC News cited an unnamed US official as saying there was concern al Qaeda might deploy suicide attackers with surgically implanted bombs to evade security.

Security was especially tight in Yemen's capital Sanaa.

Soldiers with armored personnel carriers were stationed outside buildings as police and army checkpoints went up on all the city's main thoroughfares.

“I've spent 21 years in the CIA, and I don't think I've ever seen 22 embassies closed simultaneously. This is very, very unusual,” Robert Baer, a former CIA officer in the Middle East, told CNN.

Baer said the US action comes amid an Al-Qaeda resurgence, including recent prison breaks in Libya and Iraq and turmoil in Egypt, Mali and elsewhere.

Late last week, the State Department issued a worldwide travel alert warning US citizens of possible attacks on “public transportation systems and other tourist infrastructure.”On Saturday, the global police agency Interpol issued a security alert over hundreds of militants freed in jail breaks.

Interpol said it suspected Al-Qaeda was involved in the mass breakouts in nine countries, notably Iraq, Libya and Pakistan.