WASHINGTON — A rising number of radicalized young Muslims with Western passports are traveling to Syria to fight with the rebels against the government of Bashar Assad, raising fears among US and European intelligence officials of a new terrorist threat when the fighters return home.
More Westerners are now fighting in Syria than fought in conflicts in Iraq, Afghanistan, Somalia or Yemen, according to the officials.
They go to Syria motivated by the desire to help the people suffering there by overthrowing Assad. But there is growing concern that they will come back with a burst of jihadist zeal, some semblance of military discipline, enhanced weapons and explosives skills, and, in the worst case, orders from affiliates of Al Qaeda to carry out terrorist strikes.
“Syria has become really the predominant jihadist battlefield in the world,” Matthew G. Olsen, the director of the National Counterterrorism Center, told a security conference in Aspen, Colo., this month.
He added, “The concern going forward from a threat perspective is there are individuals traveling to Syria, becoming further radicalized, becoming trained and then returning as part of really a global jihadist movement to Western Europe and, potentially, to the United States.”
Classified estimates from Western intelligence services and unclassified assessments from government and independent experts put the number of fighters from Europe, North America, and Australia who have entered Syria since 2011 at more than 600. That represents about 10 percent of about 6,000 foreign fighters who have poured into Syria by way of the Middle East and North Africa.
Most of the Westerners are self-radicalized and are traveling on their own initiative to Turkey, where rebel facilitators often link them up with specific groups, terrorism experts say. Many have joined ranks with the Al Qaeda-aligned Nusra Front, which US officials have designated as a terrorist group.
“The scale of this is completely different from what we’ve experienced in the past,” Gilles de Kerchove, the European Union’s counterterrorism coordinator, said at the conference in Aspen.
A precise breakdown of the Western fighters in Syria is difficult to offer, counterterrorism and intelligence officials said, but their estimates include about 140 French citizens, 100 Britons, 75 Spaniards, 60 Germans, and as many as a few dozen Canadians and Australians.
There are also fighters from Austria, Belgium, Denmark, Finland, Ireland, Italy, Norway, Sweden, and the Netherlands, according to a study in April by the International Center for the Study of Radicalization, a partnership of academic institutions based in London, which estimated that 140 to 600 Europeans have gone to Syria.
Only about a dozen Americans have so far have gone to fight in Syria, according to US intelligence officials. Nicole Lynn Mansfield, 33, of Flint, Mich., a convert to Islam, was killed in May while with Syrian rebels in Idlib province.
Eric Harroun, 30, a former Army soldier from Phoenix, was indicted by a federal grand jury last month on two charges related to allegations that he fought alongside the Nusra Front.
So far, terrorism analysts say, there have been no documented terrorist plots linked to European or other Western fighters returning from Syria, but France’s interior minister, Manuel Valls, called the threat “a ticking time bomb.”
Security services across Europe are stepping up their surveillance efforts and seeking ways to make it more difficult for people suspected of being jihadists to travel to Syria.
European and other Western intelligence agencies are rushing to work together to track the individuals seeking to cross the border into Syria from Turkey, although several US officials expressed frustration that Turkey has not taken more aggressive steps to stem the flow of Europeans going to fight in Syria.