LONDON - The United States will more than double its security assistance for Yemen and Britain will host an international meeting this month to seek ways of preventing the poorest Arab state from becoming an al Qaeda stronghold.

The moves highlighted mounting Western concern over Yemen after a failed Christmas Day attempt to bomb a U.S. airliner by a Nigerian man who said he had received training and equipment in the country that borders Saudi Arabia.

Al Qaeda in the Arabian Peninsula, the Yemen branch of Osama bin Laden's network, has claimed responsibility for the attempt by Umar Farouk Abdulmutallab to ignite explosives hidden in his underwear as his flight from Amsterdam approached Detroit.

Prime Minister Gordon Brown said on Friday that Yemen presented a regional and global threat as an incubator and potential safe haven for terrorism.

Brown's office said he would host a high-level meeting in London on January 28 to discuss countering radicalisation in Yemen. The talks will be held in parallel with an international conference on Afghanistan the same day.

The international community must not deny Yemen the support it needs to tackle extremism, Brown said in a statement.

The increase in U.S. backing for Yemen was announced in Baghdad by General David Petraeus, head of the U.S. Central Command.

We have, it's well known, about $70 million in security assistance last year. That will more than double this coming year, Petraeus told a news conference.

U.S. officials have said they are looking at ways to expand military and intelligence cooperation with Yemen in order to step up a crackdown on al Qaeda militants there.

But a Pentagon spokesman this week described as grossly exaggerated a report that Washington was preparing retaliatory strikes after the Detroit plane incident.

Somalia's hardline Islamist rebel group al Shabaab said on Friday it was ready to send reinforcements to al Qaeda in Yemen should the United States carry out strikes.

We call upon all Muslims to give a hand to our brothers in Yemen and we, al Shabaab, are ready to send them reinforcements ... and Inshallah (God willing) we shall win over America, said Sheikh Mukhtar Robow Abuu Mansuur, a senior official of al Shabaab, which Washington sees as an al Qaeda ally.

Somalia is separated from Yemen by the Gulf of Aden where Somali pirates have hijacked a number of international ships.

Compounding the challenge from al Qaeda, Yemen faces a separatist rebellion in the south and an insurgency by rebels from the minority Shi'ite Zaidi sect in the north.

A Yemeni government source told Reuters on Friday that 11 Shi'ite rebels, whom he described as terrorists, had been killed in clashes with the military and security forces.

The conflict, which has killed hundreds and displaced tens of thousands, drew in neighbouring Saudi Arabia in November when rebels staged a cross-border incursion into the world's biggest oil exporter.

The multiple security threats facing Yemen's government have intensified Saudi and Western concern that it could turn into an al Qaeda haven and launch pad for international attacks -- a role played by Afghanistan in the run-up to the September 11 attacks on the United States in 2001.
Yemen's Foreign Minister Abubakr al-Qirbi said this week there could be up to 300 al Qaeda militants in his country, some of whom may be planning attacks on Western targets

(Reporting by Adrian Croft in London, Jim Loney in Baghdad, Mohammed Ghobari in Sanaa and Souhail Karam in Riyadh)