US President Donald Trump has called his former Secretary of State Rex Tillerson “dumb as a rock” and “lazy as hell” a day after the former diplomat publicly said he had warned the president against doing things that would violate the law.

Mr Tillerson, who described the US leader as an undisciplined man with a disdain for reading, said Mr Trump repeatedly had to be stopped from breaking the law.

On Saturday (AEDT), Mr Trump didn’t hold back via Twitter: “Rex Tillerson didn’t have the mental capacity needed. He was dumb as a rock and I couldn’t get rid of him fast enough. He was lazy as hell.”

Mike Pompeo is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn’t have the mental capacity needed. He was dumb as a rock and I couldn’t get rid of him fast enough. He was lazy as hell. Now it is a whole new ballgame, great spirit at State!

— Donald J. Trump (@realDonaldTrump) December 7, 2018

Breaking his silence in his first public appearance in nine months, Mr Tillerson said his March 13 firing via Twitter was inevitable because he allegedly had to stop the President from violating laws, the Houston Chronicle reported.

“So often, the President would say, ‘Here’s what I want to do and here’s how I want to do it’, and I would have to say to him, ‘Mr President I understand what you want to do but you can’t do it that way. It violates the law’,” Mr Tillerson said.

“I’d say, ‘Here’s what we can do; we can go back to Congress and get this law changed. And if that’s what you want to do, there’s nothing wrong with that.’

“I told him, ‘I’m ready to go up there and fight the fight, if that’s what you want to do’.”

Mr Tillerson never specified what illegal actions the President wanted him to take.

US President Donald Trump with then Secretary of State Rex Tillerson. Image: Getty

“He got really frustrated. I think he grew tired of me being the guy who told him, ‘You can’t do that’.”

Mr Tillerson was sacked by Mr Trump as he was travelling to meet US allies in Africa. He was replaced by then-CIA director Mike Pompeo.

Mr Trump repeatedly demeaned his top diplomat, telling Mr Tillerson he was wasting his time trying to negotiate with “Little Rocket Man”, the President’s name for North Korean leader Kim Jong-un.

In addition to policy disputes, relations were strained by reports that Mr Tillerson privately called Trump a “moron”.

He went on to describe the President’s working style as undisciplined and chaotic.

“What’s challenging for me … is 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,” he said.

Rex Tillerson (right) was on a first-name basis with Vladimir Putin during his days at Exxon. Photo: AAP

“We are starkly different in our styles, we did not have a common value system.”

The former ExxonMobil CEO said Mr Trump preferred to act on his instincts rather than on evidence or advice from experts around him.

“He acts on his instincts. In some respects, that looks like impulsiveness. But it’s not his intent to act on impulse. I think he really is trying to act on his instincts.”

According to Mr Tillerson, he unsuccessfully tried to convince Mr Trump to remain in the Paris Climate Accord and the Iranian nuclear deal.

In a bombshell revelation, he also told the audience there was “no question” that the Russian government interfered in the 2016 presidential election, adding that it was thoroughly documented by intelligence agencies.

“What Russia wants to do is undermine our confidence and undermine the world’s confidence in us,” Mr Tillerson said.

Mr Trump has not replied to the allegations.

Ebony Bowden reports for New Daily from New York City.