An estimated 6,000 foreign fighters -- 600 from Europe, North America and Australia -- have entered Syria to help overthrow Syrian President Bashar Assad, The New York Times reported Sunday.
Many of those who have entered Syria travel first to Turkey, where rebel facilitators often link them up with specific groups, including the al-Qaida-aligned Nusra Front, terrorism experts say.
"Syria has become really the predominant jihadist battlefield in the world," Matthew G. Olsen, director of the National Counterterrorism Center, told a security conference in Aspen, Colo., this month. "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."
"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.
Since the start of the Syrian civil war in 2011, there have been no terror plots linked to European or other Western fighters returning from Syria -- however, French Interior Minister Manuel Valls, recently called the threat "a ticking time bomb."
In an effort to counter possible attacks, Western intelligence agencies are working together to track the people who cross the border into Syria from Turkey.