File Photo: U.S. Defense Secretary James Mattis gestures during a press briefing on the campaign to defeat ISIS at the Pentagon in Washington, U.S., May 19, 2017. (Xinhua/REUTERS)

WASHINGTON, Oct. 15 (Xinhua) -- U.S. Secretary of Defense James Mattis on Monday denied that he had registered for the Democratic Party, something that U.S. President Donald Trump suggested in an earlier interview.

"I've never registered for any political party," Mattis told reporters, adding that he had been "apolitical" since joining the Marine Corps at the age of 18.

The remark came in response to Trump's earlier comment saying that Mattis was a "sort of a Democrat" during an interview Sunday. Trump also said that Mattis "could be" leaving the administration, which Mattis also denied.

"I'm on his team. We have never talked about me leaving. And as you can see right here, we're on our way. We just continue doing our job," he told reporters.

Mattis said he had not talked to Trump after the interview had been aired.

Rumors about a potential exit for Mattis, often labeled a grown-up figure in the Trump administration, have been circulating in Washington for weeks. Mattis was put into the spotlight by a book published earlier that shed light on the inner workings of the Trump administration.

Inside the book, written by veteran journalist Bob Woodward, Mattis was said to have brushed off a directive from Trump to assassinate Syrian President Bashar al-Assad.

Although Mattis denied remarks attributed to him in the book, the impression that Mattis and Trump shared different views on certain issues lingered.

Mattis, 68, a four-star Marine Corps general until he retired in 2013, was one of the first officials to be confirmed to Trump's cabinet.