President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Monday attacked outgoing Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) and Brett McGurk, the envoy to the coalition fighting the Islamic State in Iraq and Syria (ISIS), over the Iran nuclear deal.

Trump wrote in a tweet that McGurk, who announced his resignation on Saturday, "was responsible for loading up airplanes with 1.8 Billion Dollars in CASH" and sending it to Iran as part of the nuclear deal "approved by Little Bob Corker."

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"For all of the sympathizers out there of Brett McGurk remember, he was the Obama appointee who was responsible for loading up airplanes with 1.8 Billion Dollars in CASH & sending it to Iran as part of the horrific Iran Nuclear Deal (now terminated) approved by Little Bob Corker," Trump tweeted.

For all of the sympathizers out there of Brett McGurk remember, he was the Obama appointee who was responsible for loading up airplanes with 1.8 Billion Dollars in CASH & sending it to Iran as part of the horrific Iran Nuclear Deal (now terminated) approved by Little Bob Corker. — Donald J. Trump (@realDonaldTrump) December 24, 2018

McGurk previously led a U.S. delegation that negotiated with Iran over the release of Americans detained in the country, according to The Wall Street Journal. Those negotiations led to a settlement in which the U.S. awarded $1.7 billion to Iran.

The settlement was reached more than a year after the U.S. and Iran agreed to the nuclear deal, which lifted sanctions on Iran in exchange for limits on its nuclear program. Trump earlier this year withdrew from that agreement.

McGurk first served under former President George W. Bush and stayed in the administration under former President Obama.

McGurk, who was scheduled to leave his envoy position in February, instead resigned on Saturday, reportedly in protest over Trump's plan to withdraw U.S. troops from Syria. That decision also prompted the resignation of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE.

Trump wrote in a tweet Saturday that he doesn't know McGurk and accused him of grandstanding.

Trump has also revived his feud with Corker in recent days.

After Corker said Sunday on CNN's "State of the Union" that Trump was to blame for the current partial government shutdown, Trump wrote on Twitter that Corker decided not to run for reelection this year because his poll numbers "tanked" when Trump didn't endorse him.

"Senator Bob Corker just stated that, 'I’m so priveledged [sic] to serve in the Senate for twelve years, and that’s what I told the people of our state that’s what I’d do, serve for two terms,' " Trump tweeted. "But that is Not True - wanted to run but poll numbers TANKED when I wouldn’t endorse him."

Updated at 11:49 a.m.