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 on Tuesday said that President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE expressed support for him after raising questions about Mattis's future in a weekend interview with "60 Minutes."

Reuters reported that Mattis said Trump phoned him while the secretary was in flight from Washington to Vietnam.

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"[Trump] said, 'I’m 100 percent with you,' " Mattis told reporters.

Mattis's remarks come shortly after he said that he thought "nothing at all" of Trump's comments speculating that his Defense secretary may leave the administration.

Trump said in the interview broadcast Sunday that he thinks Mattis is "sort of a Democrat" and he "may leave."

"I think he's sort of a Democrat, if you want to know the truth," the president said. "But Gen. Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves."

Mattis on Tuesday said that he's "never registered for any political party," and downplayed talk of his departure.

"I’m on his team," Mattis said. "We have never talked about me leaving. And as you can see right here, we are on our way [to Asia]. We just continue doing our job."

Mattis's potential exit has been the subject of speculation for weeks, with multiple news outlets reporting that Trump has been weighing potential replacements.

The president later denied those reports, citing Mattis's response to Bob Woodward's book, "Fear: Trump in the White House," which reported exchanges in which the Defense secretary criticizes the president to other staffers and questions Trump's understanding of key issues.

Mattis, 68, issued a statement denying he said anything attributed to him in the book, calling the work "fiction." He later told reporters not to take rumors of his exit seriously.