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 had gone to Syria.

Only about a dozen Americans have so far gone to fight in Syria, according to American intelligence officials. Nicole Lynn Mansfield, 33, of Flint, Mich., a convert to Islam, was killed in May while in the company of Syrian rebels in Idlib Province.

Eric Harroun, 30, a former Army soldier from Phoenix, was indicted in Virginia by a federal grand jury last month on two charges related to allegations that he fought alongside the Nusra Front. In February, he bragged about his involvement, posting a photo on his Facebook page saying, “Downed a Syrian Helicopter then Looted all Intel and Weapons!”

About 30 French citizens have returned from the front lines in Syria, according to Mathieu Guidère, a professor at Université Toulouse II and an expert on Islamic terrorism. He said most had been stopped by the domestic intelligence service and held for lengthy questioning under a law passed last year that allows charges to be brought for having traveled to terrorist training camps or combat zones where terrorist groups are involved.