US President Donald Trump was less prepared than Vladimir Putin for their first meeting, allowing his Russian counterpart to promote his own agenda and leaving US officials at a disadvantage, former Secretary of State Rex Tillerson has told lawmakers.

Mr Trump fired back on Twitter, saying Mr Tillerson had made up the story and calling the former Exxon CEO "’dumb as a rock’ and totally ill prepared and ill equipped to be Secretary of State."

Mr Tillerson, who was fired by Mr Trump last March, delivered his assessment of the president's approach to the talks in Hamburg in 2017, in remarks to a Congress committee Tuesday reported by the Washington Post.

"We spent a lot of time in the conversation talking about how Putin seized every opportunity to push what he wanted," a committee aide told the newspaper.

"There was a discrepancy in preparation, and it created an unequal footing."