Washington (CNN) In a rare public appearance since he was fired from office, former Secretary of State Rex Tillerson called President Donald Trump "undisciplined" and claimed that Trump would often ask him to do things unaware that such actions would violate the law.

"When the President would say, 'Here's what I want to do and here's how I want to do it.' And 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,'" Tillerson said Thursday night at a fundraiser in Houston for the MD Anderson Cancer Center, according to video of the event posted by CBS News.

He went on to add, "He got really frustrated ... I think he grew tired of me being the guy every day that told him you can't do that and let's talk about what we can do."

Tillerson's comments during a conversation with former CBS News' Bob Schieffer elicited some laughs from the audience.

His candid remarks about his former boss came nearly nine months after he was fired by Trump in March and replaced with then-CIA Director Mike Pompeo. Tillerson's exit from office followed months of tension between him and Trump.

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