Trump challenges Rex Tillerson to IQ test
He made the remark in a magazine interview when asked about reports that Mr Tillerson had called him a moron.
“I think it’s fake news,” Mr Trump told Forbes, “but if he did that, I guess we’ll have to compare IQ tests. And I can tell you who is going to win.”
Mr Trump is due to have lunch on Tuesday with Mr Tillerson.
Reports have swirled of a schism in the Trump administration between the commander-in-chief and his top diplomat, as the US faces a host of vexatious foreign policy conundrums, from North Korea to Iran.
Last week Mr Tillerson called a news conference to deny reports that he was considering quitting.
But the former ExxonMobil chief executive did not refute an NBC News report that he had called his boss a moron after a July meeting at the Pentagon.
Earlier this month, Mr Trump publicly undercut the former Texas oilman by tweeting that he was “wasting his time” trying to negotiate with nuclear-armed North Korea.
Last week the New York Times reported that Mr Tillerson was astonished at how little Mr Trump grasps the basics of foreign policy.
According to the newspaper, quoting sources close to the secretary of state, Mr Trump has been irritated by Mr Tillerson’s body language during meetings.
Mr Tillerson is said to roll his eyes or slouch when he disagrees with the decisions of his boss.
A classic Trump counterpunch
Analysis by Anthony Zurcher, BBC North America reporter
Donald Trump insists that the stories about Rex Tillerson insulting his intelligence – despite being heavily sourced – are “fake news”. Now, however, he’s lobbing one of his trademark counter-punches, just in case.
Mr Tillerson thinks he’s a moron? Well, he’s smarter than Rex, that’s for certain.
It’s classic Trump – a slightly less juvenile version of the “I guarantee you there’s no problem” retort Mr Trump snapped off during a Republican debate, when Senator Marco Rubio questioned the size of his, er, manhood.
Mr Trump tends to get touchy when people doubt his intellect. That’s probably why the “moron” line has prompted such a furious response from the White House and State Department. During the campaign he said he doesn’t have to consult generals because he has “a very good brain” and told a rally in South Carolina that he was highly educated and has “the best words”.
In August, he boasted that he was a “better student” and went to better schools than all his elite critics.
Mr Tillerson may have opened a difficult-to-repair rift with the president. While Mr Trump is quite comfortable with insult-trading, there’s one topic that’s clearly off-limits.