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US President Donald Trump has challenged his Secretary of State, Rex Tillerson, to an IQ test, in the latest sign of discord between the two.

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 had lunch on Tuesday with Mr Tillerson.

Shortly beforehand, the president maintained he still had confidence in the secretary of state.

"I did not undercut anybody," he also told reporters. "I don't believe in undercutting people."

Asked about Mr Trump's IQ test challenge, White House Press Secretary Sarah Huckabee Sanders told the daily news briefing: "It was a joke. You should get a sense of humour."

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 dismiss reports that he was considering quitting.

Media playback is unsupported on your device Media caption In October, Rex Tillerson responded to a report he called Mr Trump 'a moron'

But the former ExxonMobil chief executive did not deny 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 counter-punch

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.