* Worldwide travel alert issued last Friday, embassies closed
* Britain withdraws all embassy staff from Yemen
* Restoring stability to Yemen crucial for U.S., Gulf allies
The Air Force flew some U.S. diplomatic personnel out of Yemen on Tuesday and Washington told nationals to leave the country immediately after warnings of potential attacks that pushed the United States to shut missions across the Middle East.
Yemen, one of the poorest Arab countries, is the base for Al Qaeda in the Arabian Peninsula (AQAP), one of the most active branches of the network founded by Osama bin Laden, and militants have launched attacks from there against the West.
U.S. sources have told Reuters that intercepted communication between bin Laden's successor as al Qaeda leader, Ayman al-Zawahri, and the Yemen-based wing was one part of the intelligence behind their alert last week.
Britain, which has already advised for more than two years that its citizens in Yemen should "leave now", announced it was temporarily evacuating all its embassy staff.
Yemen is one of a handful of countries where Washington acknowledges targeting militants with strikes by drone aircraft.
In the latest strike on Tuesday, a U.S. drone fired five missiles at a car travelling in the central Maarib province killing all four of its occupants, local tribal leaders said. Yemen's state news agency Saba said four al Qaeda militants were killed in the attack.
The U.S. State Department's announcement urging Americans to leave the country follows a worldwide travel alert on Friday which prompted Washington to shut diplomatic missions across the Middle East and Africa. Some of its European allies have also closed their embassies in Yemen.
"The Department urges U.S. citizens to defer travel to Yemen and those U.S. citizens currently living in Yemen to depart immediately," the statement posted on its website said.
"On August 6, 2013, the Department of State ordered the departure of non-emergency U.S. government personnel from Yemen due to the continued potential for terrorist attacks," it added.
Previous U.S travel warnings to Yemen had also advised citizens not travel to the country, but the language of the latest announcement appeared to reflect a more imminent threat.
State Department spokeswoman Jen Psaki said U.S. relations with Yemen remained strong despite the measures taken.
"So this was, again, just to reiterate, a response to an immediate specific threat, but we continue to work with them on a number of issues," Psaki told journalists at a news briefing in Washington.
Britain also said on Tuesday it had withdrawn all staff from its embassy in the capital Sanaa, adding there was "a very high threat of kidnap from armed tribes, criminals and terrorists".
The Netherlands advised its citizens to leave Yemen as a matter of urgency, local news agency ANP reported.
France said it had not changed its previous advisory asking citizens to "be very cautious and to move around as little as possible," Helene Conway-Mouret, junior Minister for French nationals abroad, told BFM TV.
Restoring stability to Yemen - a country close to major shipping lanes and torn by regional and sectarian separatism and tribal violence as well as the al Qaeda insurgency - has been a priority for the United States.
In a statement issued in Washington, Pentagon spokesman George Little said the U.S. Air Force "transported personnel out of Sanaa, Yemen, as part of a reduction in emergency personnel" in response to a request by the State Department.
He did not specify which types of personnel were involved or where they were taken.
"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," the statement said.
Yemeni Foreign Minister Abu-Bakr al-Qirbi criticised the measures but said they would not affect relations with the United States.
"Unfortunately, these measures, although they are taken to protect their citizens, in reality they serve the goals that the terrorist elements are seeking to achieve," Qirbi told Reuters.
"Yemen had taken these threats seriously and had taken all the necessary measures to protect all the foreign missions in the country," he added.
The country's Supreme Security Committee issued a statement saying it had information al Qaeda was plotting attacks during Eid al-Fitr, this week's Muslim holiday that marks the end of the Ramadan fasting month.
The committee also published a list of 25 senior al Qaeda militants it said were being sought by security forces and offered a bounty of 5 million Yemeni riyals ($23,000) for information leading to their capture.
"Information has become available that terrorist elements of the al Qaeda network were planning to carry out terrorist acts targeting public installations and facilities, especially in a number of Yemeni provinces, in the latter days of the holy month of Ramadan and during the Eid al-Fitr holiday," it said.
Long-serving leader Ali Abdullah Saleh stepped down following months of protests against his rule in 2011, part of Arab uprisings that toppled three other heads of state. His replacement, Abd-Rabbu Mansour Hadi, met U.S. President Barack Obama in Washington last week.
Yemen is home to 56 of the 86 detainees still being held in Guantanamo Bay, Cuba, and security in Yemen is a key element of any deal to send them back so that Obama can fulfil a pledge to close the U.S. prison camp.
Washington's warnings last week concerned possible attacks in the region, based on intelligence including intercepted communication between al Qaeda leaders. Some officials pinpointed Yemen as the main concern.
No figures on the number of Americans in Yemen were immediately available. Washington had consistently cautioned citizens against travelling to Yemen since the protests in early 2011 that eventually forced Saleh to step down.
Al Qaeda's Yemen-based branch AQAP has been behind plots against Western targets and neighbouring Saudi Arabia. It claimed responsibility for a failed attempt by a Nigerian man to blow up a Detroit-bound trans-Atlantic airliner with explosives hidden in his underwear on Christmas Day, 2009.
The United States has acknowledged killing Anwar al-Awlaki, an American citizen and al Qaeda preacher, in a drone strike in Yemen in September 2011. The Obama administration's policy allowing the killing of a U.S. citizen has been controversial.