Thousands of foreign fighters have bolstered the ranks of militant groups in Syria and Iraq in recent months, according to U.S. officials, driving fresh concern about potential terrorist plots aimed at the United States or its allies.
The surge, as well as intelligence that Yemeni terrorists have developed a powerful cellphone bomb designed to avoid detection at airports, is behind the Obama administration’s increasingly urgent warnings that a European or American passport holder might try to take down a passenger jet or plan other deadly mayhem.
As many as 10,000 foreign fighters — a third more than in February — have joined the militias seeking to overthrow Syrian President Bashar Assad and the Islamic State fighters who have swept across northern Iraq, said a counter-terrorism official who spoke on condition of anonymity to discuss classified assessments.
As many as 3,000 of them hold European or other Western passports and thus can travel easily across most borders. Several dozen, perhaps as many as 100, hold U.S. passports, and officials say that number is growing.