WASHINGTON -- U.S. President Donald Trump responded to criticism from his former secretary of state Rex Tillerson on Friday by calling Tillerson "dumb as a rock" and "lazy as hell."

During a rare public appearance in Houston on Thursday evening, Tillerson, the former Exxon Mobil CEO, discussed his difficult tenure working for Trump. He called the president "undisciplined" and said the president "doesn't like to read, doesn't read briefing reports, doesn't like to get into the details of a lot of things."

Tillerson also said Trump frequently asked him to do things that he had to explain were illegal or otherwise ill-advised.

It was an extraordinary rebuke from a man Trump once said would be "a star!" at the State Department.

And it was met with an even more biting response from Trump on Friday. The president tweeted that Tillerson "didn't have the mental capacity needed for the job" and lavished praised on Tillerson's replacement, Mike Pompeo.

"Mike Pompeo is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn't have the mental capacity needed," Trump wrote.

"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!"

Last year, Trump apparently had a higher opinion of Tillerson's abilities, calling him "our wonderful secretary of state" and declaring that "we work well together."