CIA Director John Brennan said Friday that the Islamic State had “snowballed” beyond Iraq and Syria, estimating that at least 20,000 fighters from more than 90 countries have gone to join the militant group, several thousand of them from Western nations, including the United States.

Brennan’s statement marks a change from the narrative the Obama administration has been pushing on the success of the fight against ISIS.

“Left unchecked, the group would pose a serious danger not only to Syria and Iraq, but to the wider region and beyond, including the threat of attacks in the homelands of the United States and our partners,” Brennan told the Council on Foreign Relations in New York.

He added that ISIS takes advantage of new technology to “coordinate operations, attract new recruits, disseminate propaganda and inspire sympathizers across the globe to act in their name.”

The CIA director said that the terror group has ballooned in size to about 20,000 members, and points to the recent pledge of allegiance to ISIS by Nigeria-based Boko Haram.

“This will be a long-term struggle,” he said. “If there is one thing we have learned over the years, it is that success against terrorism requires patience and determination.”

Brennan’s comments about ISIS’ staying power differs from claims made by top Obama administration officials as recently as last month assessing the progress against the terror organization.

Secretary of State John Kerry and retired General John Allen have both made public statements about ISIS in the past. In February, Allen said that half of the group’s leaders in Iraq had been killed. Kerry took it a step further and extended the claim to Syria as well.

Experts, though, say the claims of success are overinflated.

“We currently don’t have a percentage attached to that statistic,” Army Captain John J. Moore, a spokesman for U.S. Central Command, told Bloomberg View.

Others, like Princeton University scholar of Near Eastern Studies Cole Bunzel, told Bloomberg claims of killing half of ISIS leadership is difficult to make because the terror organization’s “leadership” is subjective.

He added that he was “very skeptical” that 50 percent of the group’s leaders were killed because ISIS has publicly announced when senior members had been killed in the past but has not made such an announcement about someone in its core leadership being killed in the past five years.

Michael Smith, a principal at the counterterrorism consulting group Kronos Advisory LLC, largely echoed Bunzel’s comments and told Bloomberg “Jihadi groups typically eulogize slain leader,” and so far, there hasn’t been any evidence that high-ranking members had been killed.