Washington (CNN) Former Secretary of Defense James Mattis warned "ISIS will resurge" if the US does not keep the pressure on, days after the White House announced US forces in northern Syria would pull back in advance of a planned Turkish military offensive.

"In this case, if we don't keep the pressure on, then ISIS will resurge. It's absolutely a given that they will come back," Mattis told NBC in a clip released Saturday by the news organization ahead of Sunday's "Meet the Press." Mattis resigned in December after President Donald Trump's plans to withdraw troops from Syria became public. The retired four-star general was privately adamant in urging Trump against the pullback.

"ISIS is not defeated," Mattis told NBC. "We have got to keep the pressure on ISIS so they don't recover."

The move to pull back troops from the area ahead of the Turkish military offensive drew widespread criticism , including from top Republicans like Senate Majority Leader Mitch McConnell. It was a major shift in US foreign policy and some lawmakers and the Syrian Democratic Forces have argued it effectively gave Turkey the green light to attack US-backed Kurdish forces, though Trump administration officials have argued Turkey would have invaded regardless of the presence of US troops.

Kurdish-led SDF, which has long been considered one of America's most reliable partners in Syria, has played a key strategic role in the campaign against ISIS in the region.