Donald Trump calls Rex Tillerson 'dumb as a rock'

US President Donald Trump has blasted former secretary of state Rex Tillerson in a sharply critical tweet, a day after the nation's former top diplomat publicly recounted that the President had tried to do things in a way that violated the law, ABC News reported.

In an interview with CBS News, Mr Tillerson described Mr Trump as "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".

Asked how his relationship with Mr Trump had gone off the rails, Mr Tillerson said it might have reflected what he saw as his obligation to tell the President that he could not do certain things because they were illegal or would violate US treaties. "I'd have to say to him, well Mr President, I understand what you want to do, but you can't do it that way. It violates the law, it violates treaty … you know, he got really frustrated."

Mr Trump took to Twitter to return fire, saying Mr Tillerson "didn't have the mental capacity needed" for the job, and "was dumb as a rock".

"I couldn't get rid of him fast enough. He was lazy as hell," he said.