* Flow of Western militants going to war zones picks up
* Al Qaeda prizes Western radicals as they can evade capture
* In Europe, Britain and Germany are most at risk
(Adds U.S. drone strike killing 8 Germans)
By William Maclean, Security Correspondent
LONDON, Oct 4 (Reuters) -U.S. and British alerts about possible attacks in Europe highlight concern that growing numbers of militants are going from the West to remote war zones for training in answer to al Qaeda&${esc.hash}39;s online call for violence.
The immediate trigger for Sunday&${esc.hash}39;s travel alerts was intelligence about a plot against European targets reportedly originating with a group of individuals in mountainous northern Pakistan, some of them believed to be European citizens.
Few details of the conspiracy are known. But the plot appears to be of the kind that Western officials believe poses the most significant danger today -- the use of so-called self-radicalised militants with no previous record of extremism.
Al Qaeda&${esc.hash}39;s leadership, increasingly restrained by missile strikes from U.S. drones in northwest Pakistan, prizes such "home-grown" recruits as they have Western passports and can travel overseas easily, experts say.
Some experts suspect the flows to countries like Pakistan and Yemen of would-be militants have risen especially in the past two years, despite stepped-up efforts by some Western governments to counter Islamist radicalisation among Muslim minority communities.
"It&${esc.hash}39;s a serious phenomenon in Europe, especially in Britain and Germany," said Edwin Bakker, a security and conflict expert at the Netherlands Institute of International Relations.
"The motivation is not always simply Islamist extremism, sometimes individuals also go to find a sense of purpose for themselves, or for excitement."
Pakistani intelligence officials said a suspected U.S. drone strike killed eight militants of German nationality in northwest Pakistan on Monday. Two missiles from a suspected CIA pilotless aircraft struck a mosque in Mirali in North Waziristan.
The strike came a day after the U.S. State Department issued an alert warning American citizens to exercise caution while travelling in Europe. Britain raised the terrorism threat level to "high" from "general" for its citizens travelling to Germany and France. [ID:nLDE6920E7]
The plot that triggered the alerts involved al Qaeda and allied militants, possibly including European citizens or residents, intelligence sources said last week. They said the militants were plotting coordinated attacks on European cities.
"SERIOUSLY DANGEROUS PEOPLE"
The use of Western-based militants radicalised in online chat-rooms or through email contacts with hardline preachers has grown sharply in the last two years.
Among the most dramatic of the mostly failed attacks and plots in the West was the failed bombing of New York&${esc.hash}39;s Times Square by Pakistani-born U.S. citizen Faisal Shahzad on May 1.
Joerg Ziercke, head of Germany&${esc.hash}39;s BKA Federal Crime Office, said last month more than 400 Islamist radicals were living in Germany, some of whom had been trained in camps overseas, including a hard core with combat experience in Afghanistan. Police had seen a rise in German residents visiting the camps.
European Union counter-terrorism coordinator Gilles de Kerchove told Reuters on Friday the plot showed the continent had to do more to impede extremists going overseas to train.
"There&${esc.hash}39;s a number, a not insignificant number, of seriously dangerous people going around," he said. [ID:nLDE69026C]
"For some time there has been serious concern about people born or resident in Europe travelling to jihadi conflict zones -- Afghanistan-Pakistan, Somalia, Yemen."
In a report to EU member governments in June, Kerchove said al Qaeda remained the EU&${esc.hash}39;s biggest threat and urged better control of the movements of potential terrorists in the EU.
In Britain, Jonathan Evans, head of the MI5 security service, said last month there was a serious risk of a lethal attack and militants in Yemen and Somalia were growing threats.
Many of the militants who travel out to Pakistan, often Europeans of south Asian, Arab or Turkish ancestry, intend to go over the border to Afghanistan and fight Western forces there.
But al Qaeda has frequently sought to persuade them to go back home and attack Western targets there, experts say.
These efforts have had some success.
THREAT "QUICKLY GROWING"
Paul Cruickshank, an alumni fellew at the Center on Law and Security at New York University&${esc.hash}39;s School of Law, said in a report that in the majority of 21 serious militant plots against the West since 2004, plotters either received direction from or trained with al Qaeda or its allies in Pakistan.
Between 100 and 150 Westerners were suspected of going to the region for armed training in the last year, his Feb. 2010 study reported Western counterterrorism officials as saying.
Some of those who travel are struggling novices, who view the trips as rites of passage, said an Oct. 1 joint report by the Homeland Security Policy Institute at America&${esc.hash}39;s George Washington University and the Swedish National Defence College.
Others are diehard militants seeking mortal combat and martyrdom against non-Muslim militaries, it said.
While hard numbers on the flow of foreign fighters to conflict zones were kept secret by Western spy agencies, it was evident that "the threat from foreign fighters is now quickly growing in size and prominence", the report said.
A Sept. 10 report by U.S. terrorism experts Bruce Hoffman and Peter Bergen for the Bipartisan Policy Center said the threat from al Qaeda had grown more complex, and U.S. citizens and residents were playing an increasing role in the leadership of al Qaeda planning and operations.
The group&${esc.hash}39;s ability to carry out a major attack like that of Sept. 11 was "far less formidable" than it was in 2001. But the group might be able to bomb symbolic American targets such as the subways of Manhattan in attacks that would kill dozens.
"This level of threat is likely to persist for years to come," it said. (Editing by Mark Trevelyan)
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