In unprecedented comments, former Secretary of State Rex Tillerson said President Donald Trump would often ask him to violate US law.

Trump's former cabinet member called the president "undisciplined" in an interview at a fundraiser in Houston, Texas.

The former ExxonMobile Corporation executive also said "there's no question" that Russia interfered in the 2016 presidential election.

At his first public appearance in almost nine months, Rex Tillerson, former secretary of state under President Donald Trump, said the president often asked Tillerson to violate US law.

The surprising comments were made to Bob Schieffer of CBS News at a fundraiser for the MD Anderson Cancer Center in Houston, Texas. The former cabinet member's remarks about the president, including his assertions that the president frequently wanted to violate US law, are unprecedented.

Tillerson also took a strong stance against "very opportunistic" Russian President Vladimir Putin, and said "there's no question" that Russia interfered in the 2016 presidential election.

Regarding his relationship with Trump, Tillerson said he and the president did not have a "common value system."

"When the president would say, 'Here's what I want to do and here's how I want to do it,' I'd 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. It violates a treaty,'" Tillerson said in the interview. "I think he grew tired of me being the guy every day that told him, 'You can't do that.'"

Trump would often grow frustrated at those conversations, the former secretary said. Tillerson's experience at what he described as "highly disciplined, highly process-oriented" ExxonMobil Corporation made it difficult to transition to the Trump White House, he said.

"It's challenging ... to go to work for a man who is pretty undisciplined," he said.