In a new assessment, the U.S. intelligence community judges that large numbers of foreigners fighting for Islamic State in Iraq and Syria likely will stay to defend what is left of their self-declared caliphate rather than return to their homelands, a top U.S. counterterrorism official said on Friday.

"Many if not most of the foreign fighters who made their way to the conflict zone will end up staying, fighting and potentially dying in order to maintain the caliphate," Nicholas Rasmussen, the director of the U.S. National Counterterrorism Center, told the annual Aspen Security Forum.

That contrasts with the previous assessment that many foreign fighters will return home, posing major security threats.