WASHINGTON - Some 75 staffers at the US embassy in Yemen have been evacuated aboard a US military plane because of a region-wide terror threat, a US official said Tuesday.The plane, accompanied by a support aircraft, flew to the US air base in Ramstein, Germany, the official told AFP on condition of anonymity.A Pentagon official declined to comment, citing security concerns.The Pentagon said earlier it was flying staffers out of Yemen in response to a request from the State Department amid worries over a possible al-Qaeda attack after a message from its leader urging an attack was reportedly intercepted."The US Department of Defense continues to have personnel on the ground to support the US State Department and monitor the security situation, Pentagon spokesman George Little said in a statement.The State Department also urged Americans not to travel to Yemen and urged those there to leave right away.Britain said Tuesday it had evacuated all staff from its embassy in Yemen.An AFP correspondent in the capital Sanaa said that concrete barriers surrounding the British embassy have been raised higher as part of increased security measures across the capital.A US-flagged passenger plane landed at Sanaa airport, the correspondent said, possibly to evacuate US nationals. Residents meanwhile reported that two drones had overflown the capital. The US is the only nation known to be operating drones within Yemen.The alert, in which the State Department said it had ordered all non-essential staff out of Yemen, came hours after a drone strike killed four Al-Qaeda militants there and two days after the closure of some two dozen embassies in the Middle East and Africa.Intercepts between Zawahiri and Nasser al-Wuhayshi, the leader of Al-Qaeda's Yemen affiliate, sparked the closure of the US missions overseas and an earlier worldwide travel alert, US media reported. The New York Times said late 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 this past Sunday.CNN meanwhile reported that Zawahiri told Wuhayshi to "do something," causing officials in both Washington and Yemen to fear an attack was imminent.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 early 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 Ramadan 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.The US closure list includes 15 embassies or consulates that were shut on Sunday -- the fifteenth anniversary of Al-Qaeda's attacks on US embassies in East Africa -- as well as four additional posts.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 that 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.The posts to be closed include Abu Dhabi, Amman, Cairo, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Sanaa, Tripoli, Antananarivo, Bujumbura, Djibouti, Khartoum, Kigali and Port Louis.New closures were announced in Madagascar, Burundi, Rwanda and Mauritius. The outposts that are reopening include those in Afghanistan, Algeria, Bangladesh, Mauritania, Iraq and Israel.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.Residents said they heard the sound of a drone overhead, which could only be American as Washington is the sole power to operate the unmanned aircraft in the region."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.