President Trump fired back Friday at former Secretary of State Rex Tillerson, calling him "dumb as a rock" after the ex-diplomat claimed he always had to remind the president some of his plans would “violate the law.”

In a rare public appearance, Tillerson sat down with former CBS News journalist Bob Schieffer during a fundraiser for the MD Anderson Cancer Center in Houston, according to the Houston Chronicle.

Tillerson blamed his strained relationship with Trump on the fact they did not share a “common value system” and were “obviously starkly different in our styles.”

“When the president would say, ‘Here's what I want to do, and here's how I want to do it,’ and I'd have to say to him, ‘Well, Mr. President, I understand what you want to do, but you can't do it that way. It violates the law, it violates the treaty, you know. He got really frustrated,” Tillerson said. “I think he grew tired of me being the guy every day who told him he can’t do that and let’s talk about what he can do.”

He said he told Trump he would be willing to “fight the fight” to change laws in Congress so the president could press forward with his agenda.

Trump fired back on Twitter, saying Tillerson "didn't have the mental capacity needed" as the nation's top diplomat.

"He was dumb as a rock and I couldn’t get rid of him fast enough," Trump said. "He was lazy as hell. Now it is a whole new ballgame, great spirit at State!"

Tillerson, 66, was fired from the White House in March after rumors of a rift between him and Trump had been circulating for some time. In leaving, Tillerson said Washington, D.C., can be a “very mean-spirited town.”

EX TILLERSON’S CAREER AS SECRETARY OF STATE UNDER TRUMP

Tillerson also contended Trump tends to act “on his instincts” although it’s “not his intent to act on impulse.”

“What was challenging for me – coming from the disciplined, highly process-oriented ExxonMobil corporation – to go to work for a man who is pretty undisciplined, doesn’t like to read, doesn’t read briefing reports, doesn’t like to get into the details of a lot of things, but rather just kinda says, ‘Look, this is what I believe,’” Tillerson said.

FORMER TRUMP ADMINISTRATION OFFICIALS: A LIST OF NOTABLE DEPARTURES

During the event, Tillerson, who is from Texas, said there is “no question” Russia interfered in the 2016 election. He said the country seeks to “undermine our confidence and undermine the world’s confidence in us,” according to the Houston Chronicle.

Trump announced Tillerson’s firing as the nation’s top diplomat in a March tweet which named Mike Pompeo as his replacement.

“Thank you to Rex Tillerson for his service!” Trump said then.