The U.S.-led military strategy to defeat the Islamic State group in Iraq and Syria will create a cheaper, nimbler and lethally dangerous version of the terror network in terms of its ability to attack Western targets, top counterterror officials said Tuesday.

"The so-called caliphate will be crushed," FBI Director James Comey told members of Congress Tuesday morning, adding, "Through the fingers of that crush will come hundreds of very dangerous people."

"There will be a terrorist diaspora some time in the next two to five years like we've never seen before."

Comey testified before the Senate Committee on Homeland Security and Governmental Affairs alongside Jeh Johnson, the secretary of Homeland Security, and Nicholas Rasmussen, the director of the National Counterterrorism Center – two organizations founded in the wake of the Sept. 11 attacks to prevent another such incident.

Sen. Ron Johnson, the committee's chairman and a Wisconsin Republican, observed a moment of silence to commemorate the 15th anniversary of the attacks before gaveling in the hearing on how the American counterterror infrastructure has improved since that time.

Tuesday's session comes as the U.S. says much of the territory once controlled by the Islamic State group, also known as ISIS or ISIL, has been wrested back. Now the coalition it leads is setting its sights on its most difficult tasks so far: the liberation of Mosul in Iraq and of Raqqa in Syria, which form the axis of the Islamic State group's symbolic heartland. Military and administration officials have indicated both campaigns likely will begin this fall and will require thousands of local troops to complete.

Comey, however, warned that upon success, "when ISIL is reduced to an insurgency and those killers flow out, they will try to come to Western Europe and they will try to come here."

And Rasmussen, tasked with coordinating intelligence and military efforts to track extremist organizations, said the Islamic State group's ability to launch attacks has not been diminished. He added that destroying the extremist network's so-called religious caliphate also will require hunting down and dismantling the infrastructure it uses to recruit disaffected Muslims and move them toward conducting attacks.

"The shrinkage of the physical caliphate has been such a high, first-order priority of our strategy to defeat ISIL," he said. 'There was going to be a lag between the time where we achieved territorial success on the battlefield and the time at which we could actually succeed in constraining ISIL's ability to attack overseas."

"It puts us in a period of sustained vulnerability none of us is comfortable with."

The Islamic State group has had to devote expenses and resources toward maintaining the territory it controls, operating schools, courts and other services.