Donald Trump has called Rex Tillerson “dumb as a rock” on Twitter shortly after the former secretary of state made a controversial statement about his working relationship with the president during a rare public appearance.

“Mike Pompeo is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn’t have the mental capacity needed,” the president wrote Friday afternoon. “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 ball game,” he continued, adding, “Great spirit at State!”

The tweet followed comments Mr Tillerson made at a fundraiser for the MD Anderson Cancer Centre in which he said, “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."

The former secretary of state also asserted the Russian Kremlin was responsible for interfering in the 2016 presidential elections, saying “there’s no question” on the issue based on evidence obtained by US intelligence agencies.

“What Russia wants to do is undermine our confidence and undermine the world’s confidence in us,” said Mr Tillerson, who held zero political experience prior to Mr Trump appointing him secretary of state but had maintained connections with Russian officials as an executive at Exxon.

Prior to his work in the state department, Mr Tillerson received the Order of Friendship from Russian President Vladimir Putin, one of the highest distinctions the Kremlin gives to foreign citizens.

On Friday, however, the former Cabinet member said “I will be honest with you, it troubles me that the American people seem to want to know so little about issues that they are satisfied with a 128 characters,” while describing how he would supposedly be forced into working out legal resolutions for Mr Trump’s allegedly illegal proposals in real-time.

“I’d say here’s what we can do. We can go back to Congress and get this law changed,” Mr Tillerson continued. “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.”

His comments, arriving nearly 10 months after the president seemingly fired him from his top position at the State Department over Twitter, marked one of the first interviews he’s provided since leaving public office about his time in Washington.

Speaking with reporters shortly after firing Mr Tillerson in March, Mr Trump said he respected the former secretary’s “intellect,” adding he “got along well with Rex.”

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In the controversial tweet effectively announcing his dismissal, Mr Trump wrote of Mr Tillerson, “Mike Pompeo, Director of the CIA, will become our new Secretary of State. He will do a fantastic job! Thank you to Rex Tillerson for his service! Gina Haspel will become the new Director of the CIA, and the first woman so chosen.”

“Congratulations to all!” he added.

Mr Trump’s Friday comments sparked immediate backlash online from his high-profile critics, including Michael McFaul, the former ambassador to Russia under Barack Obama, who responded to the tweet writing, “Whenever I think that I can no longer be shocked by the president's vulgar ways, he ups the ante.”

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The controversial public exchange also arrived amid explosive new developments surrounding Special Counsel Robert Mueller’s investigation into Russian interference in the 2016 election. Friday marked a deadline for the special counsel and New York prosecutors to release a memo outlining cooperation contributed to numerous probes by Michael Cohen, the president’s former lawyer and longtime fixer.