The number of Americans travelling to Syria to fight in the civil war there has increased in recent months and remains a major concern to US law enforcement and intelligence officials, said FBI Director James Comey.
Comey said the FBI is worried that the Americans who have joined extremist groups allied with al-Qaeda in Syria will return to the United States to carry out terrorist attacks.
Comey declined to give a precise figure for Americans believed to be involved in the Syrian struggle but said the numbers are "getting worse".
"I said dozens last time," said Comey, referring to an interview with reporters four months ago. "It's still dozens, just a couple more dozen."
A senior US counterterrorism official estimated this year that 60 to 70 Americans have travelled to fight in Syria. Comey said that Americans in Syria are actively recruiting other Americans to join the fight.
Comey said the threat associated with foreign fighters in Syria is of concern not only to the US but also is "a huge focus" of European intelligence officials.
"It's the first thing we talk about when I go visit a counterpart," said Comey, who has visited 13 FBI legal attaché offices abroad since he became director in September.
Comey said thousands of fighters are travelling to Syria from European countries, and they are a focus for the FBI because many of them could easily get into the United States.
"They're visa-waiver countries," Comey said. "If someone flows out of Syria, they can flow in here very easily."