A Turkish-baked Syrian fighter fires during clashes in the border town of Ras al-Ain | Nazeer Al-Khatib/AFP via Getty Images Mattis: ISIS will surge back after Trump’s Syria withdrawal ‘It’s absolutely a given that they will come back,’ ex-Pentagon chief says.

Former U.S. Defense Secretary James Mattis said ISIS "will resurge" in Syria, in his first public comments following U.S. President Donald Trump's withdrawal of troops from the region.

"I think Secretary of State Pompeo, the intelligence services, the foreign countries that are working with us have it about right that ISIS is not defeated. We have got to keep the pressure on ISIS so they don't recover," Mattis said in an interview on NBC News' "Meet the Press" set to air Sunday.

Mattis' warning comes after Trump announced last week that the U.S. will move special operation teams away from the Syrian border as Turkey launches an offensive against Kurdish forces. The Kurds have long fought alongside the U.S. against the Islamic State in the region.

"You can pull your troops out, as President Obama learned the hard way, out of Iraq but the 'enemy gets the vote' as we say in the military. And in this case, if we don't keep the pressure on then ISIS will resurge. It's, it's absolutely a given that they will come back," Mattis continued.

Trump's move was widely lambasted by both parties in Congress. A bipartisan group of House lawmakers is drafting a resolution to impose punitive sanctions on Turkey for the move.

Late Friday, a U.S. military outpost came under fire as Turkish military launched air and artillery strikes. Defense Secretary Mark Esper warned Turkey that they must protect U.S. military personnel.

Turkey’s military said Saturday it captured Ras al-Ayn, one of the largest border town in Syria, marking one of the biggest gains since the invasion began, according to the Associated Press.

Mattis resigned in December 2018 days after Trump announced the pullout of American troops from Syria, a decision that was partly reversed.

Trump has long called for drawing down the U.S. military's commitments in the Middle East and around the world, making the issue a key part of his presidential campaign.

"The Endless Wars Must End!" Trump tweeted Saturday afternoon.

Trump later called into "Justice with Jeanine" on Fox News, saying that Turkish President Recep Tayyip Erdoğan was going into Syria whether or not troops were withdrawn.

"He was going to go in anyway. They’ve been fighting the Kurds for 200 years. He was going in anyway," Trump told host Jeanine Pirro.