WASHINGTON — President Donald Trump challenged Secretary of State Rex Tillerson to “compare IQ tests,” delivering a sharp-edged ribbing that threw a bright spotlight on his seemingly shaky relationship with his top diplomat. The White House insisted the president was only joking.

Trump issued the challenge in an interview with Forbes magazine, when asked about reports that Tillerson called him a “moron” after a classified briefing this summer. The president responded that if the claim was true, the two should duke it out in a battle of brainpower.

“And I can tell you who is going to win,” Trump said.

The White House and the State Department suggested Tuesday that the president was simply trying to make light of what they describe as inaccurate reports of tension. But coming amid increasingly public signs of strain between the president and Tillerson, the remark landed with a distinct hint of malice.

Trump’s comments have threatened to undermine Tillerson’s diplomatic initiatives and sow confusion among allies and foes over whether he speaks for the U.S. and has the support of the White House. That uncertainly could impact a number of foreign policy crises, including the nuclear threat posed by North Korea and the imminent decision to be made as to whether to continue the Iran nuclear pact.

Trump on Tuesday declared that he had confidence in Tillerson just hours after the publication of the interview — and before a private luncheon with Tillerson and Defense Secretary Jim Mattis.

But people close to Trump say the president has grown increasingly dissatisfied with the former Exxon CEO, whom he views as holding a merely conventional view of America’s role in the world and lacking star power. Tillerson, meanwhile, is said to have grown weary of Trump contradicting his public pronouncements and of becoming increasingly isolated in a capital to which he has never warmed.

The NBC News report last week claiming Tillerson described Trump as a “moron” — to associates after a highly classified July briefing — brought the simmering frustration between the men into the open.

This account is based on conversations with several White House aides, State Department officials and others who spoke with the two men over the past week.

Seldom backing down from a fight, Trump escalated another public feud on Tuesday, unloading on ‘Liddle Bob Corker,” a Republican senator who has dubbed the White House an “adult day care center” and said the president could be setting the nation on the path toward World War III.

Other GOP leaders urged the two men to calm a quarrel that could imperil the Republican agenda on Capitol Hill and lawmakers’ election chances next fall.

As for Tillerson, Trump had no relationship with him before last year’s election but offered him the secretary of state post after being impressed with his global oil tycoon resume and receiving recommendations from foreign policy heavyweights including former Secretary of State Condoleezza Rice and former Secretary of Defense Bob Gates.