In a new jab at his own secretary of state, President Trump threatened to compare his IQ with Rex Tillerson's following reports the secretary of state privately called him a "moron."

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

Trump's comments come after the Washington Post reported the president has shown "flashes of fury" in an outburst following Tillerson's comment, which in turn reportedly sent White House Chief of Staff John Kelly scrambling to manage Trump's flare ups.

Trump has repeatedly appeared to undermine Tillerson, tweeting negatively about the secretary of state after a dialogue was reportedly reopened with North Korea. But Trump doesn't see it that way, telling Forbes, "I'm not undermining … I think I'm actually strengthening authority."

NBC News White House Correspondent Peter Alexander reported that both Tillerson and Secretary of Defense Jim Mattis will have lunch with President Trump Tuesday at the White House.

Last week, NBC News reported that Tillerson called Trump a "moron" and threatened to resign during a meeting at the Pentagon in July. Tillerson, in a hastily arranged press conference, pushed back against the report but did not deny that he used the word "moron" to describe the president, casting questions about it aside as "petty stuff."

The president blasted NBC News over the report, dubbing the story "fake news" and even suggested the Senate Intelligence Committee should investigate the network for what he deemed "made up" news reports.

But NBC News has staunchly defended its reporting. NBC News correspondent and MSNBC anchor Stephanie Ruhle, one of four reporters who worked on the Tillerson story, offered an impassioned defense of her team's reporting last week, and went after Trump specifically after he complained the network didn't verify their report with him.

"Sir, we didn't need you to verify that he called you a ' moron,' " Ruhle said. "He did it behind your back."

"We have a very good relationship," Trump told reporters on Saturday about Tillerson. "We disagree on a couple of things. Sometimes I'd like him to be a little bit tougher. But other than that, we have a very good relationship."