Former national security adviser Susan Rice ripped 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 late Saturday for declaring he doesn't know his top envoy in the fight against the Islamic State in Iraq and Syria (ISIS), saying that she "can assure you" former President Obama "knows him well."

"The fact that you say you don’t know @brett_mcgurk speaks volumes about your commitment to fighting ISIS @realDonaldTrump. Why don’t you know the man who has done more than any civilian to degrade ISIS? I can assure you @BarackObama knows him well," tweeted Rice, who was Obama's national security adviser for most of his second term.

The fact that you say you don’t know @brett_mcgurk speaks volumes about your commitment to fighting ISIS @realDonaldTrump. Why don’t you know the man who has done more than any civilian to degrade ISIS? I can assure you @BarackObama knows him well. https://t.co/OzvKRrmVF5 — Susan Rice (@AmbassadorRice) December 22, 2018

The envoy, Brett McGurk, resigned on Friday because he reportedly disagreed with Trump's decision to withdraw troops from Syria, which Trump claimed he was doing because the U.S. had defeated ISIS in that region.

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McGurk had reportedly planned to leave the position in February but decided to resign early. His resignation came a day after Defense Secretary James Mattis also resigned.

Trump in a tweet on Saturday suggested McGurk was being a "grandstander" and wrote that he doesn't know him.