Sen. Chris Murphy (D-Conn.) is expressing concerns after President Trump tweeted that he did "not know" his top envoy in the fight against the Islamic State in Iraq and Syria (ISIS), Brett McGurk.

"Uhhhh...you don't know your own coordinator of the anti-ISIS campaign?" Murphy tweeted late Saturday. "Over the past 5 years, no one has done more to put ISIS on its heels than Brett. We all know and rely on him."

"The fact that our President has no clue who Brett is should scare the hell out of every American."

McGurk abruptly resigned on Friday over disagreements with Trump's decision to pull U.S. troops out of Syria.

Trump criticized McGurk as a "grandstander" over the departure, but said in a tweet that he did not know the official.

"Brett McGurk, who I do not know, was appointed by President Obama in 2015," the president 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 was reportedly planning to leave the administration in February, submitted his resignation soon after Defense Secretary James Mattis announced his own resignation over differences with Trump on his Syria strategy.

Trump's plan to withdraw the 2,000 troops currently in Syria has rattled Pentagon officials and lawmakers. McGurk had criticized the hypothetical move as "reckless."