FBI director James Comey has said that more than 200 Americans have travelled or attempted to travel to Syria to fight for Islamic State (Isis).
"We continue to identify individuals who seek to join the ranks of foreign fighters ... and also homegrown violent extremists who may aspire to attack the United States from within," Comey told the Senate Select Committee on Intelligence on 8 July.
Earlier, he had urged technology companies to allow securities authorities access to encrypted communications to help combat the threat of radicalisation among young Muslim Americans.
Speaking to a Senate Judiciary Committee hearing, Comey said that Isil is urging its supporters through Twitter to carry out attacks.
"The tools we are asked to use are increasingly ineffective," Comey said. "ISIL says go kill, go kill...we are stopping these things so far...but it is incredibly difficult. I cannot see me stopping these indefinitely."
Deputy Attorney General Sally Yates, also speaking at the committee, rejected the notion that the government was seeking backdoor access to encrypted communications, reported Reuters.
"We are not seeking a front door, back door, or any kind of door...but we are seeking to work with the industry," Yates said. She urged Congress to work with Silicon Valley and said they were looking to tailor solutions to individual companies.