Former 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 called President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE an “unusual president” in his first televised interview since leaving office.

In an interview set to air Sunday, Mattis insisted to CBS News he “will not speak ill of a sitting president.”

“I’m not going to do it,” Mattis said in the interview for “CBS Sunday Morning,” according to a preview released Friday.

“He’s an unusual president, our president is,” Mattis added. “And I think that especially with the — just the rabid nature of politics today, we’ve got to be careful. We could tear this country apart.”

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Mattis also said as part of his first major public remarks this week that he hasn’t spoken with Trump since he resigned in December.

The former Defense chief has been speaking out this week ahead of the release of his new book, “Call Sign Chaos: Learning to Lead.”

The book, which comes out Tuesday, is not expected to be a tell-all of his time in the administration.

But an excerpt published in The Wall Street Journal on Wednesday included implicit criticisms of Trump.

“Nations with allies thrive, and those without them wither,” Mattis wrote. “Alone, America cannot protect our people and our economy. At this time, we can see storm clouds gathering. A polemicist’s role is not sufficient for a leader. A leader must display strategic acumen that incorporates respect for those nations that have stood with us when trouble loomed.”

Mattis did not mention Trump by name in that part of the passage. But when Mattis resigned as Defense secretary in December, his resignation letter to Trump stressed his differences with the president on the importance of allies.

Mattis resigned after Trump announced a since-reversed plan to withdraw all U.S. troops from Syria, a move Mattis and others feared would leave U.S. partners such as the Kurds open to attack.

Even as Mattis has recently taken veiled shots at Trump, he has refused to directly criticize the president. In an interview with The Atlantic published Thursday, Mattis said he owed the administration a “duty of silence.”

"When you leave an administration over clear policy differences, you need to give the people who are still there as much opportunity as possible to defend the country,” Mattis said.

“There is a period in which I owe my silence,” he added. “It’s not eternal. It’s not going to be forever.”