Trump challenges Secretary of State Rex Tillerson to 'compare IQ tests' “And I can tell you who is going to win," the president said.

 -- President Donald Trump challenged his secretary of state to compare IQ tests, suggesting he would score higher than Rex Tillerson amid reports that Tillerson called the president a “moron.”

“I think it’s fake news, but if he did that, I guess we’ll have to compare IQ tests,” Trump said in a new interview with Forbes magazine. “And I can tell you who is going to win.”

The president sat down for lunch Tuesday with Tillerson and Secretary of Defense James Mattis at the White House. Tillerson and Trump also previously met on Friday to discuss a national security issue, the White House confirmed to ABC News.

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.

Tillerson called Trump a "moron" after a meeting with national security advisers in July, NBC News reported last week, which Tillerson’s office has denied.

"The secretary does not use that type of language,” State Department spokeswoman Heather Nauert said last week. “The secretary did not use that type of language to speak about the president of the United States. He does not use that language to speak about anyone ... he did not say that.”

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.