Former FBI Director James Comey expressed his disappointment with former Secretary of Defense Jim Mattis for not speaking out against President Trump since he left the administration.

Comey, who has been a vocal critic of Trump since being fired in May 2017, said he would be a "coward" for not speaking out and questioned the former defense secretary's reluctance to denounce Trump or his policies.

After calling himself "an admirer of Jim Mattis as a leader" during a book conversation in New York City on Monday, Comey said, "I don't get his explanation for why he isn't speaking about what he knows."

"He doesn't have to share confidential, classified conversations with the president," Comey continued. "But you are a civilian leader who has a perspective that would be useful to the American people. What's the reason you're not sharing that?"

Mattis, 69, resigned from office last year. His departure reportedly was a result of disagreements with Trump over U.S. policy in Syria. Mattis didn’t support drawing back troops from the region, while Trump did.

While on a book tour this fall, the retired four-star Marine Corps general described what he called a "duty of silence" upon leaving the administration in order to preserve national security while Trump is in office. “We have to give the people who are protecting us some time to carry out their duties without me adding my criticism to the cacophony that is right now so poisonous," Mattis said in an interview with the Atlantic.

Mattis, who explained it would be improper to "endanger the country by attacking the elected commander in chief," also said he would not be silent forever.

This is not to say Mattis has been completely unwilling to take swipes at Trump. After the president was said to have called Mattis "the world’s most overrated general," he ribbed the commander in chief during a joke-filled speech at the white-tie Al Smith Dinner in October.

"I guess I'm the Meryl Streep of generals, and frankly, that sounds pretty good to me," Mattis said, referring to the Academy Award-winning actress. "And you do have to admit that, between me and Meryl, at least we've had some victories."

By comparison, Comey has been very open about his disapproval of Trump, who, in turn, called Comey's behavior treasonous. He leaked memos of his conversations with the president, in what he has described as an effort to spark a special counsel investigation. This prompted a stern rebuke from the Justice Department inspector general, who found Comey "violated" FBI rules.

Comey, 58, also wrote a book, A Higher Loyalty, which was very critical of Trump while calling for "ethical leadership."

Comey's critique of Mattis took place during a sit-down discussion with New York Times columnist James Stewart, who recently had a book published: Deep State: Trump, the FBI, and the Rule of Law.

"None of us who are worried about the integrity of this country, it's intelligence judgments, and the rule of law should hesitate to say that's not okay. Even if people are going to say, 'Oh, you're a partisan pundit.' Rod Rosenstein has called me a 'partisan pundit.' I don't care. We can't be cowed or numbed into silence," Comey said, referring to the deputy attorney general who wrote a memo cited in his firing.

"Just because the conduct is so outrageous in a way that increases — I don't love this phase of my life, as I've told you. But I would be a coward if I didn't speak out when I know things I can say, I have a perspective, and I have a platform. I want this part of my life to go away, and I'd like it to go away soon," he added, to cheers.