Absence does not make the heart grow fonder when it comes to US President Donald Trump and his first secretary of state, Rex Tillerson.

Nine months after Trump summarily dismissed his top diplomat by tweet, he followed it up with another tweet on Friday in which he branded Tillerson "dumb as a rock" and "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

The biting words came after Tillerson, in an appearance at a charity event in Houston, said their relationship soured over Trump's repeated demands that Tillerson do things that were illegal.

"So often, the president would say, 'Here's what I want you to do, and here's how I want you to do it,' " Tillerson said at a fundraiser for the MD Anderson Cancer Center in Houston, in his first public remarks about his truncated tenure since he was let go in March.

"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,' " he said.

As if to confirm how toxic their interactions had become, Trump praised Tillerson's successor, Mike Pompeo, and dismissed the abilities of Tillerson, who ran Exxon Mobil before stepping down to work for Trump.

"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!"

While at the State Department, Tillerson privately fought back against many of the budget cuts the White House enforced on him, but ultimately made deep cuts in staffing that contributed to low morale as veteran diplomats were shown the door or made to feel unwelcome and unvalued.

In his remarks, Tillerson did not offer any specific examples of which requests he deemed illegal. He said he offered to work to change the law, but that apparently did not curb Trump's frustration.

"I'd say, 'Here's what we can do,' " Tillerson said. " '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.' "

Tillerson noted that he had never met Trump before Vice President Pence invited Tillerson to the White House. At the end of his meeting with Trump, Tillerson said, he was offered the job as the nation's top diplomat.

Tillerson also took a swipe at Twitter - not the president's use of it, but the short attention span it has helped engender in many Americans.

Saying Trump was elected using modern-day tools to tap into strong emotions, he added, "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 128 characters.

"I don't want that to come across as a criticism of him. It's really a concern I have about us as Americans, and us as a society, and us as citizens."

Tillerson was fired a few hours after returning from a trip to Africa. Though he had been forewarned that Trump was unhappy with him, Tillerson learned of his dismissal through a tweet in which Trump congratulated his new pick for the job, Mike Pompeo, and concluded with a breezy, "Thank you to Rex Tillerson for his service!"

Tillerson could not immediately be reached for comment on Trump's latest slam against him.