Brett McGurk, the U.S. envoy for the global coalition to defeat Islamic State (ISIS), has resigned in the wake of President Trump’s decision to pull U.S. troops out of Syria, Fox News confirmed Saturday -- coming on the heels of the resignation of Defense Secretary Jim Mattis earlier this week.

U.S. officials said this week that the Trump administration is making plans to pull all 2,000 troops out of Syria, and Trump later tweeted that “we have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.”

Ground troops moved into Syria in 2015 to combat ISIS in the region, amid an escalating Syrian civil war. Trump had talked about pulling out of Syria before, but military leaders had warned about ISIS re-emerging should the U.S. do so. There is also concern about abandoning the U.S.' Kurdish allies.

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On Thursday, Trump announced that Mattis was retiring in February. Mattis’ resignation letter said that Trump had a right to a Defense Secretary “whose views are better aligned with yours” on policy. Sources told Fox News this week that Mattis' resignation was "in protest" over the president's national security policies and that more resignations could be coming.

The Associated Press reported that McGurk said in a resignation letter to Secretary of State Mike Pompeo that ISIS was on the run, but wasn’t yet defeated and that U.S. work in Syria wasn’t yet done. Fox News has confirmed that McGurk submitted his resignation letter on Friday, a day after Secretary Mattis resigned. His resignation is effective December 31. His resignation was first reported by CBS News.

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The president reacted to the news via Twitter Saturday evening, saying he had neither met McGurk nor appointed him.

"Brett McGurk, who I do not know, was appointed by President Obama in 2015. Was supposed to leave in February but he just resigned prior to leaving," Trump tweeted. "Grandstander? The Fake News is making such a big deal about this nothing event!"

"If anybody but your favorite President, Donald J. Trump, announced that, after decimating ISIS in Syria, we were going to bring our troops back home (happy & healthy), that person would be the most popular hero in America," he continued in a follow-up tweet. "With me, hit hard instead by the Fake News Media. Crazy!"

McGurk, who was an Obama appointee in 2015, had said in a Dec. 11 press conference that it would be "reckless" to consider ISIS defeated and that the “enduring defeat of a group like this means you can’t just defeat their physical space and then leave.”

“Nobody is saying that they are going to disappear. Nobody is that naive,” he told reporters. “So we want to stay on the ground and make sure that stability can be maintained in these areas.”

An official told the Associated Press that McGurk was planning to leave the job in mid-February, but felt he could no longer continue after Trump’s decision and Mattis’ resignation.

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McGurk previously served as a deputy assistant secretary of state for Iraq and Iran, and led secret side talks with Iran about the release of American prisoners during the negotiations for the Iran nuclear deal.

McGurk had previously worked as a Supreme Court law clerk to the late Chief Justice William Rehnquist. He had also served on former President George W. Bush’s National Security Council staff, where in 2007 and 2008, he was the lead U.S. negotiator on security agreements with Iraq.

McGurk’s departure is the latest sign of criticism from Trump’s allies in his administration, and in Congress, for the decision.

Sen. Lindsey Graham, R-S.C. recently blasted Trump for his decision to pull troops out of Syria, calling it an "Obama-like decision" -- an allusion to Obama's decision to pull troops from Iraq in 2011. In return, Trump fired back, saying it was difficult to understand why Graham would be against saving countless lives.

"So hard to believe that Lindsey Graham would be against saving soldier lives & billions of $$$. Why are we fighting for our enemy, Syria, by staying & killing ISIS for them, Russia, Iran & other locals? Time to focus on our Country & bring our youth back home where they belong!" Trump tweeted.

On Saturday, Trump said that ISIS is “largely defeated” and that other countries, such as Turkey could take care of whatever is left.

“We’re coming home!” he tweeted.

Fox News' Nicholas Kalman, Jennifer Griffin, Ben Florance, Elizabeth Zwirz and The Associated Press contributed to this report.