The FBI and CIA directors say that despite the group's loss of territory, Islamic State militants will continue to pose serious national security problems for the United States and Europe.

CIA Director John Brennan said at an intelligence conference Thursday in Washington that many fighters who don't die on the battlefield will try to return to their home countries, where they could launch IS-inspired attacks.

FBI Director James Comey says if the IS caliphate is crushed, many surviving militants — individuals he described as "hardened killers" — will flow into western Europe and try to duplicate the Paris and Brussels attacks or try to bring the fight to the United States.

Comey says the U.S. must help its European allies share intelligence among themselves and with the United States.