The U.S. Embassy said it had received a threat by al Qaeda, which U.S. intelligence agencies believe has a growing presence in the poor Arab country.

A British Foreign Office spokeswoman cited security reasons for the embassy's closure but declined to say if any specific threat had been made.

Yemen has already tightened security on its coastline to stop Islamist militants infiltrating from Somalia and held talks with a U.S. general on strengthening cooperation.

But Foreign Minister Abubakr al-Qirbi said his government was not coordinating strikes against al Qaeda with the United States.

Western allies have sought to bolster Yemen's government, which faces facing a Shi'ite rebellion in the north and a separatist movement in the south, for fear that al Qaeda might exploit its instability to launch more attacks across the globe.

A Nigerian man, charged with trying to bomb a Detroit-bound passenger plane on Christmas Day, is believed to have received training from the militant group in Yemen.

Al Qaeda said the attempt was in retaliation for U.S. involvement in Yemen and its military support for the government, which has launched an offensive against the militants.

The U.S. Embassy told its Yemeni staff to stay in their homes on Sunday.

"The U.S. Embassy in Sana'a is closed today, January 3, 2010, in response to ongoing threats by Al-Qaeda in the Arabian Peninsula (AQAP) to attack American interests in Yemen," a statement on the embassy website said.

Spain's embassy in Yemen has restricted access but remains open, the Spanish government said. El Mundo newspaper said the embassy would be closed to the public on Monday.

In Washington, a senior aide to President Barack Obama said the United States has indications al Qaeda was planning an attack against a target in Sanaa.

"We know that al Qaeda is out there. We know we have to mind our steps," homeland security and counterterrorism aide John Brennan told CNN.

U.S. officials have said Washington was looking at ways to expand military and intelligence cooperation with Yemen to increase pressure on al Qaeda militants in the Arabian Peninsula.

The United States and Britain have agreed to fund a counter-terrorism police unit as part of the effort.

NO AGREEMENT ON STRIKES

Foreign Minister Qirbi, quoted by the state news agency, said Yemen was cooperating with foreign countries in exchanging information and training.

Asked if Yemen had agreed to allow U.S. missiles and aircraft to strike al Qaeda targets in Yemen, Qirbi said: "There is no agreement with the United States in this regard.

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U.S., U.K. Close Yemen Embassies