Rasmussen told the Senate Homeland Security Committee that he expects the ISIS group, after losing its physical territory, to become a covert operation that will still conduct and inspire attacks around the world.
"There is not, in fact, a direct link between ISIS' battlefield position in Iraq and Syria and the group's capacity to inspire external attacks," he said.
"The ISIS ability to reach globally is still largely intact," said Rasmussen, who has been director of the National Counterterrorism Center since 2014.
In recent months US-led coalition forces have expelled ISIS fighters from its key Iraq strongholds of Mosul and Tal Afar and they are close to eliminating it in Raqa, Syria.
This has forced the remnants of the ISIS terrorists down into the middle Euphrates River valley where a last-stand siege is expected.
But Rasmussen said the group is and will continue to be able to recruit followers around the world, ready to undertake attacks.
When its defeat on the battlefield is final, he said, US terror experts expect it to revert to the form ir took in an earlier incarnation as the Al-Qaeda in Iraq insurgency of 2004 to 2008.
"Winning on the battlefield in places like Mosul and Raqa is a necessary but an insufficient step in the process of eliminating the ISIS threat to our interests," he said.
"It's simply going to take longer than we would like to translate victory on the battlefield into a genuine threat reduction."
Foreign fighters threat
Rasmussen, director of the US National Counterterrorism Center, also said that the number of foreign ISIS fighters returning back to their home countries is not as great as had been feared.
"It is not nearly as large in volume as perhaps we anticipated. That's a good thing, that we're not going to have to deal with thousands and thousands of foreign fighters departing the conflict zone."
However, he added, "I would say though that quality matters here, in some ways more than quantity."
"The wrong set of individuals who escape from the conflict zone in Iraq and Syria, if they have got a particularly specialized set of skills, or a particularly full Rolodex, or deep connections into an extremist community in of Europe or potentially here inside the United States, they could pose a significant threat to us."
He said that Al-Qaeda, which has found refuge in Syria's Idlib province under the name Hayat Tahrir al-Sham, remains a potent threat despite being overshadowed by the ISIS group.
Sixteen years after stunning the United States with its September 11, 2001 attacks, Al-Qaeda is "a strikingly resilient organization" that is still able to recruit followers, maintain relationships between affiliated groups and raise money.
Both groups, Rasmussen said, maintain a focus on renewed attacks against western aviation, as evidenced by the recent foiled attempt to bomb an aircraft in Australia, he said.
That case "shows that terrorists are aware of security procedures. They watch what we do and they try to learn from it," he said.
"It suggests that the bad guys have the ability to adapt their tactics in attempt to defeat airport security measures."
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