Trump challenges Secretary of State Rex Tillerson to 'compare IQ tests'

“And I can tell you who is going to win," the president said.

Prior to Tuesday's lunch, Trump said that he did not "undercut" Tillerson and that he had confidence in his secretary of state during an Oval Office appearance before reporters with former Secretary of State Henry Kissinger.

The sentiment was echoed by White House press secretary Sarah Sanders at Tuesday's press briefing. She added that the IQ test comment was not meant to imply that Tillerson was not bright, but rather made in jest.

"The president certainly never implied never implied that the secretary of state was not incredibly intelligent. He made a joke, nothing more than that," said Sanders, who added that the pair "had a great visit" earlier in the day.

Trump has repeatedly dismissed the accusation as “fake news” but NBC News is standing by its reporting.

Tillerson reaffirmed his support for Trump last week in a statement to rebut the report, but would not answer whether he ever used the word “moron” to describe the president.

The State Department has also attempted to swat down rumors of Tillerson's possible resignation in July.

At the time, tensions between the White House and Tillerson were allegedly high over staffing at the State Department and Tillerson's ability to make decisions about the nation's foreign policy agency, senior administration officials told ABC News.

ABC News' Jordyn Phelps contributed to this report.

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