Former Defense Secretary James Mattis described his ex-boss, President Trump, as "unusual" but was reluctant to criticize him during an interview that came amid a series of statements suggesting prior tension between the two.

"I will not speak ill of a sitting president. I'm not going to do it," Mattis told CBS during an interview scheduled to air Sunday. "He's an unusual president, our president is. And I think that especially with, just the rabid nature of politics today, we gotta be careful. We could tear this country apart."

Mattis also revealed that he hasn't spoken with the president since resigning late last year. During his time in the administration, rumors swirled around Washington that Mattis had sometimes clashed with Trump.

Recent reports like the CBS one -- his first televised interview since leaving -- do seem to offer some insight into the president's disagreements with his chief military adviser.

Mattis' departure in 2018 prompted speculation as it occurred just after the president announced he would withdraw from Syria.

MATTIS SWIPES AT TRUMP, WARNS OF 'STORM CLOUDS GATHERING' AMID FRACTURED ALLIANCES

Mattis reportedly saw the decision as betraying America's allies. "This is how I saw the strength of America, that we keep our alliances together and we keep them tight," Mattis reportedly said. "And if I wasn't the right person to do this, the president needed someone more aligned with his views."

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News of those comments came just a day after The Atlantic reported that Mattis told Trump that he would lose the war against ISIS. “You’re going to have to get the next secretary of defense to lose to ISIS. I’m not going to do it," he said.

The Wall Street Journal, on Wednesday, also published comments that appeared to be implied digs at the president.

In The Journal's essay, Mattis warned about a polemicist serving as president. “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," he said.

He also noted his concern with "internal divisiveness," and suggested he resigned after his attempts to persuade Trump stopped working.

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“When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution,” Mattis said.

Fox News' Brooke Singman contributed to this report.