WASHINGTON - President Donald Trump has dismissed reports that Rex Tillerson once called him a "moron" - but just to make it clear that he's smarter than his secretary of state he has suggested taking IQ tests to prove it.

Last week, Tillerson was forced to deny an NBC News report that he had called the president a moron after a July meeting at the Pentagon, holding a news conference to insist: "He's smart."

Nevertheless, something about the incident still seems to bother Trump, who was due to have lunch with Tillerson at the White House later on Tuesday. "I think it's fake news," Trump told Forbes magazine. "But if he did that, I guess we'll have to compare IQ tests. And I can tell you who is going to win."

The report came just days after Trump upbraided the former ExxonMobil CEO for "wasting his time" trying to negotiate with North Korea. The Twitter rebuke revived rumours that Tillerson is unhappy at hs post, but he insists he has no intention of resigning.