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 al-Assad, raising fears among American 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 Mr. 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 the roughly 6,000 foreign fighters who have poured into Syria by way of the Middle East and North Africa.