United States President Donald Trump on Friday said former Secretary of State Rex Tillerson was "dumb as a rock".

Heaping praises on current Secretary of State Mike Pompeo for doing a "great job", Trump took a potshot at Tillerson, asserting that he did not have the necessary "mental capacity" and 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!" the US President tweeted.

Trump's scathing remarks on Tillerson came a day after the fired Secretary of State claimed that the US President was "undisciplined" and tried to break the law.

"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,'" Tillerson said at a fundraising event in Houston on Thursday.

Underlining the fractious relationship he shared with Trump, he further 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.' He's so undisciplined, unread and willing to break the law."

This is not the first time that Tillerson has sparred over Trump since he got fired eight months ago. He reportedly called the US President a "moron" after a meeting at the Pentagon and refused to take back his remark.

In March, Trump dismissed Tillerson over a disagreement on various issues, including matter related to North Korea and the Iran nuclear deal.

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