WASHINGTON — Islamic extremist groups in Syria with ties to Al Qaeda are trying to identify, recruit and train Americans and other Westerners who have traveled there to get them to carry out attacks when they return home, according to senior American intelligence and counterterrorism officials.

These efforts, which the officials say are in the early stages, are the latest challenge that the conflict in Syria has created, not just for Europe but for the United States, as the civil war has become a magnet for Westerners seeking to fight with the rebels against the government of President Bashar al-Assad. At least 70 Americans have either traveled to Syria, or tried to, since the civil war started three years ago, according to the intelligence and counterterrorism officials — a figure that has not previously been disclosed.

The director of the F.B.I., James B. Comey, said Thursday that tracking Americans who have returned from Syria had become one of the bureau’s highest counterterrorism priorities.

“We are focused on trying to figure out what our people are up to, who should be spoken to, who should be followed, who should be charged,” Mr. Comey said in a meeting with reporters, without referring to specific numbers. “I mean, it’s hard for me to characterize beyond that. It’s something we are intensely focused on.”