The FBI also has formed a special team to identify US citizens traveling to Syria to fight with anti-government groups, and is working with both intelligence and law enforcement agencies to investigate such individuals, a spokesman said.
US security agencies have expressed growing anxiety for months about a steady stream of Western militants heading to Syria. Most of the foreigners join up with the most extreme factions trying to oust Syrian President Bashar Assad, including Jabhat al-Nusra and the Islamic State of Iraq and the Levant.
US and allied officials say their main fear is that veteran fighters, radicalized by their Syrian experience, will launch terror attacks once they return home. Authorities in Western Europe say they have uncovered plots by fighters returning from Syria.