The United States and Britain closed their embassies in Yemen on Sunday over security concerns about possible militant attacks after the failed bombing of a US-bound plane on Christmas Day.

The US Embassy said it had received a threat by al Qaeda, which US 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 US general on strengthening co-operation.

But Foreign Minister Abubakr al-Qirbi said his government was not co-ordinating 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 US involvement in Yemen and its military support for the government, which has launched an offensive against the militants.

The US Embassy told its Yemeni staff to stay in their homes on Sunday. 'The US 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 intests in Yemen,' a statement on the embassy website said.

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.

US 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.

Anti-terror unit

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

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 US missiles and aircraft to strike al Qaeda targets in Yemen, Qirbi said: 'There is no agreement with the United States in this regard.'

British Prime Minister Gordon Brown's office said Britain and the United States had agreed to intensify their joint work to tackle 'the emerging terrorist threat' from both Yemen and Somalia in the wake of the failed plane attack.

Washington has already increased training, intelligence and military equipment provided to Yemeni forces, helping them to stage raids against suspected al Qaeda hideouts last month.

General David Petraeus, head of the US Central Command, said last week that Washington would more than double its $70 million security assistance to Yemen.