President Trump has grown frustrated with Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE over his perception that the top diplomat is more interested in burnishing the United States' reputation abroad than Trump's own image, The Washington Post reported Wednesday.

The relationship between the two men has remained tense for months. According to the Post, Trump and Tillerson have butted heads over major policy issues, including the decision to send more troops to Afghanistan, and small annoyances, like the secretary's habit of not returning Trump's phone calls.

Trump has also been irritated by the fact that Tillerson has advocated for a more traditional approach to foreign policy, the Post reports. The secretary of State, for example, has suggested a more diplomatic approach to North Korea and has voiced support for the U.S. to remain a party to the Iran nuclear deal.

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In an unusual move on Wednesday, Tillerson held a news conference, in which he sought to quash reports that his relationship with Trump is on shaky ground.

In particular, he addressed an NBC News report claiming that he had threatened to quit over the summer and had to be talked out of it by top administration officials, including Vice President Pence.

"The vice president has never had to persuade me to remain as secretary of State because I have never considered leaving," Tillerson said.

Tillerson, however, did not address the report's claim that the secretary of State had once referred to Trump as a "moron" after a particularly heated meeting on the administration's strategy in Afghanistan.

Heather Nauert, a spokeswoman for the State Department, later denied that Tillerson had used such language to describe the president, and insisted that the two men maintained a strong working relationship.

Still, Tillerson and Trump have reportedly been frustrated with each other for months, and the president has appeared, at times, to undercut his top diplomat on foreign policy issues.

Over the weekend, after Tillerson had signaled that the U.S. had direct lines of communication with North Korea, Trump took to Twitter to say that Tillerson was "wasting his time" trying to open up a dialogue with Pyongyang.