Former Secretary of Defense James Mattis criticized President Trump’s decision to remove American troops from Syria, saying in his first public comments on the matter that the Islamic State now “will resurge” in 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,” Mattis said during an interview on NBC’s “Meet The Press.” “We have got to keep the pressure on ISIS so they don't recover."

Mattis’ comments come after Trump announced that he was pulling troops from Syria, greenlighting an invasion by Turkey of the war-torn region and stirring up strong bipartisan criticism from lawmakers in Washington that he was endangering regional stability and risking the lives of Syrian Kurdish allies who brought down the Islamic State group in Syria.

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"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,” Mattis said. “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 was also critical of Trump’s decision to abandon support of Kurdish fighters in the region, arguing that it will make it difficult for Washington to make allies in the future. The Kurdish-led Syrian Democratic Forces was the main U.S. ally in the fight and lost 11,000 fighters in the nearly five-year battle against ISIS.

“Reinstilling trust will be difficult for Americans at this point,” he said.

Turkish President Recep Tayyip Erdogan said Friday that Turkey won't stop until the Syrian Kurdish forces withdraw at least 20 miles from the border.

The Turkish military aims to clear Syrian border towns of Kurdish fighters' presence, saying they are a national security threat. Since Wednesday, Turkish troops and Syrian opposition fighters backed by Ankara have been advancing under the cover of airstrikes and artillery shelling.

Mattis’ comments differ strongly from those of the man who took his former job.

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Defense Secretary Mark Esper on Sunday defended the president’s call to remove U.S. troops from Syria – saying the move will be done “as safely and quickly as possible” – but admitted that the situation was growing more worrisome by the minute.

Esper said he spoke to Trump on Saturday night amid growing signs that the Turkish invasion, which began Wednesday, was growing more dangerous.

"In the last 24 hours, we learned that they (the Turks) likely intend to expand their attack further south than originally planned — and to the west," Esper said.

The U.S. also has come to believe that the Kurds are attempting to "cut a deal" with the Syrian Army and Russia to counter the invading Turks, he said. As a result, Trump "directed that we begin a deliberate withdrawal of forces from northern Syria," Esper said.

Trump, in a tweet Sunday, said: "Very smart not to be involved in the intense fighting along the Turkish Border, for a change. Those that mistakenly got us into the Middle East Wars are still pushing to fight. They have no idea what a bad decision they have made. Why are they not asking for a Declaration of War?"

Esper said he was aware of reports of hundreds of ISIS prisoners escaping as a result of the Turkish invasion and of atrocities being committed against Syrian Kurds by members of a Turkish-supported Syrian Arab militia.

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"It gets worse by the hour," Esper said. "These are all the exact things" that U.S. officials warned Erdogan would likely happen by ignoring U.S. urgings not to invade northern Syria.

Esper said there was "no way" U.S. forces could have stopped the Turks, who assembled a force of about 15,000 troops on the Syrian border, supported by air power.

"We did not sign up to fight Turkey, a longstanding NATO ally, on behalf of the (Kurdish-led Syrian Democratic Forces). This is a terrible situation," he said.

The Associated Press contributed to this report.