The Spanish embassy restricted access to its premises on Sunday, but remained open. The El Mundo newspaper said the embassy would be closed to the public on Monday.

John Brennan, the assistant for homeland security and counterterrorism to Barack Obama, the US president, meanwhile told CNN's "State of the Union" programme that "there are indications that al-Qaeda is planning an attack against a target in Sanaa".

Hakim al-Masmari, the editor-in-chief of the Yemen Post newspaper, told Al Jazeera that the closure can only mean "that they believe al-Qaeda threat is very serious".

"If you look at the recent video tapes from al-Qaeda, they clearly mention they have no enemy in Yemen except the US interests in Yemen. They warned the Yemeni soldiers against helping the Americans in any such way," al-Masmari said.

"We really expect attacks on the US interests in the next month by al-Qaeda."

Joint support

The US move came shortly after the British government announced plans to join the United States in funding an "anti-terrorist" force in Yemen.

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Gordon Brown, the British prime minister, has said he will hold a meeting in London on January 28 to discuss how to counter radicalisation in Yemen.

British officials also said that Brown and Obama believed more peacekeepers were needed to curtail the crisis in Somalia, which is located across the Gulf of Aden from Yemen.

The US government has said it will be more than doubling its military assistance to Yemen.

Obama on Saturday blamed al-Qaeda in the Arabian Peninsula, which claims to be affiliated with Osama bin Laden's organisation, for the attempted bombing of a US airliner bound for the city of Detroit on Christmas Day.

"We're learning more about the suspect. We know that he travelled to Yemen, a country grappling with crushing poverty and deadly insurgencies," the US president said on Saturday.

"It appears that he joined an affiliate of al-Qaeda and that this group, al-Qaeda in the Arabian Peninsula trained him, equipped him with those explosives and directed him to attack that plane headed for America."

US military aid to Yemen has been inconsistent in recent years, with Sanaa receiving $4.3m in 2006, up to $26m in 2007, down to nothing in 2008, and back up still higher, to $67m in 2009.

Long-term problem

Mahan Abedin, director of the Center for the Study of Terrorism, says that while US aid has fluctuated, the al-Qaeda presence in Yemen has not.

Al-Qaeda fighters in Yemen have previously threatened attacks on US interests
"They've had Yemen in their sights for a very long time ... at least as long as September 11. Even prior to that … Yemen has had a strong militancy problem since the late 1980s."

The fear among Western governments is that Yemen is poised to collapse amid extreme poverty and dwindling resources, and the ungoverned state would provide a safe haven for armed groups.

Gordon Brown, the British prime minister, has said he will hold a meeting in London on January 28 to discuss how to counter radicalisation in Yemen.

The government in Sanaa is struggling to contain an uprising by Shia Houthi rebels in the country's north and a secessionist movement in the south, in addition to the re-grouping of al-Qaeda fighters in recent years.

General David Petraeus, the commander of the US central command, visited Yemen on Saturday to discuss military and economic co-operation with Ali Abdullah Saleh, the Yemeni president, according to officials from both countries.

Petraeus had announced a day earlier that US military aid to Yemen "will more than double this coming year".

Limited options

Experts say Obama's military options to combat al-Qaeda in the Arabian Peninsula are limited.

David Newton, a former US ambassador to Yemen, said his main option "is one he's already doing - using predators [drones] to go after terrorists that they are able to locate".

"To do that, [he] has to co-operate with Saudi security and Yemeni security, and [use] our own independent means to find them."

Newton said the greater challenge will be to stabilise the country, which has seen a failing economy, severe water shortages, and a ballooning population in recent years.

Other al-Qaeda affiliated groups, such as in Somalia, have meanwhile vowed to support the Yemeni fighters.