President Donald Trump tweeted Saturday evening that he didn't know Brett McGurk, the top US official leading the fight against ISIS.

Trump called McGurk a "grandstander" for resigning over Trump's recent decision to withdraw 2,000 troops from Syria.

McGurk reportedly told his colleagues in an email on Friday that he could not in good conscience carry out Trump's orders to withdraw 2,000 troops.

Defense Secretary Jim Mattis also resigned this week, citing differences between his and Trump's views on how to treat US allies and adversaries.

President Donald Trump on Saturday said he didn't know Brett McGurk, the top US official leading the fight against the Islamic State group, and called him a "grandstander" for resigning over Trump's decision to withdraw troops from Syria.

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

McGurk, who served as the special presidential envoy to the 79-nation Global Coalition to Defeat ISIS, was indeed set to retire in February. But on Friday he announced he would resign early.

He told his colleagues in an email obtained by The New York Times that he could not in good conscience carry out Trump's orders to withdraw 2,000 troops.

"The recent decision by the president came as a shock and was a complete reversal of policy that was articulated to us," McGurk said in the email. "It left our coalition partners confused and our fighting partners bewildered."

Read more: Top US official leading fight against ISIS resigns with scathing letter calling Trump's Syria troop withdrawal a 'complete reversal' of US policy

Brett McGurk, the special presidential envoy for the Global Coalition to Defeat ISIS. Bram Janssen/AP

He continued: "I worked this week to help manage some of the fallout but — as many of you heard in my meetings and phone calls — I ultimately concluded that I could not carry out these new instructions and maintain my integrity."

McGurk's resignation came one day after the resignation of Defense Secretary Jim Mattis, who wrote a blunt letter telling Trump he had "the right to have a Secretary of Defense whose views are better aligned with yours" on issues such as respecting US allies and condemning its enemies.

Trump's decision to withdraw the troops triggered condemnation across the political spectrum, with many Republicans denouncing the decision out of fear that such a move would invigorate US foes and pave the way for an ISIS resurgence.

Read more: Trump's move to pull US troops out of Syria was reportedly the final straw for Mattis

Defense Secretary Jim Mattis answers a reporter's question about the ambush of U.S. troops in Niger before a meeting with Israeli Defense Minister Avigdor Lieberman at the Pentagon, Thursday, Oct. 19, 2017, in Washington. Associated Press/Alex Brandon

But in another tweet on Saturday night, Trump accused the media of unfairly criticizing him over the Syria decision.

"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 said. "With me, hit hard instead by the Fake News Media. Crazy!"

Trump also took a shot at Mattis, tweeting that he gave Mattis a "second chance" after former President Barack Obama fired him as the head of Central Command.

"Interesting relationship-but I also gave all of the resources that he never really had," Trump said. "Allies are very important-but not when they take advantage of U.S."