President Trump's special envoy to the fight against the Islamic State in Iraq and Syria (ISIS), Brett McGurk, shortly before his resignation last week said the terror group "will remain a threat for some time."

"We’re on track now over the coming months to defeat what used to be the physical space that ISIS controlled,” McGurk told CNBC four days before Trump announced that he would begin pulling all U.S. troops from Syria. “That will not be the end of ISIS.”

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“Nobody is naive,” McGurk told the news outlet at a security forum in Doha, Qatar. “The small clandestine cells, the individual terrorist attacks, will remain a threat for some time. That is why we have to remain together as a global coalition to keep the pressure on.”

McGurk, the special presidential envoy for the Global Coalition to Defeat ISIS at the State Department, submitted his resignation on Friday due to his disagreement with the president's decision to withdraw U.S. troops from Syria.

He was reportedly planning to the leave the administration in February, but accelerated his departure following the president's surprise announcement.

McGurk told Secretary of State Mike Pompeo of his resignation late Friday. He is expected to resign on Dec. 31.

Defense Secretary James Mattis announced his own resignation from Trump's Cabinet shortly before McGurk, also citing policy differences following Trump's announcement.

Days before his resignation, McGurk told reporters that he thought it would be "reckless" to pull the U.S. out of the region.

“I think it's fair to say Americans will remain on the ground after the physical defeat of the caliphate, until we have the pieces in place to ensure that that defeat is enduring," McGurk told reporters at the State Department.

"It would be reckless if we were just to say, well, the physical caliphate is defeated, so we can just leave now. I think anyone who's looked at a conflict like this would agree with that," he added.