Rex Tillerson. Andrew Harrer-Pool/Getty Images "I don't know. I'm not a diplomat."

US Secretary of State Rex Tillerson made that admission during a meeting with Russian and Iranian foreign ministers at the United Nations headquarters late last month.

Tillerson had gathered officials involved in the 2015 nuclear deal with Iran in what turned out to be a sort of airing of US grievances related to the Obama-era agreement. The New Yorker's Dexter Filkins outlined the exchange in a report set to be published in the magazine later this month.

In the meeting, Tillerson echoed some of President Donald Trump's broad reservations about the nuclear deal. He accused Iran of funding militant groups, supporting Syrian President Bashar Assad, and antagonizing American troops in the Persian Gulf, according to the report.

Iranian foreign minister Javad Zarif pushed back, saying the US had not held up its promise to lift certain financial sanctions against Iran. Tillerson was resolute, but ultimately seemed to back off from the exchange, Filkins wrote, with Tillerson suggesting that US relations with Iran were mired in generations of baggage. "Maybe we don't have it in our capacity to change the nature of this relationship," Tillerson said according to the report. "Maybe we leave it for the next generation to try."

Tillerson added: "I don't know. I'm not a diplomat."

President Donald Trump and Secretary of State Rex Tillerson during a bilateral meeting. AP Photo/Alex Brandon

Indeed, the former ExxonMobil CEO had not previously served in any diplomatic capacity before Trump appointed him as secretary of state. Tillerson spent much of his professional life working for the oil giant.

The experience has been challenging amid months of news reports that Tillerson and Trump's working relationship was tenuous, and that Tillerson had considered resigning on more than one occasion.

The job only seemed to get tougher in the last two weeks as Trump publicly undermined Tillerson's efforts to open a dialogue with North Korea, and culminating in reports that Tillerson had called Trump a "moron" during a meeting with the president's national security team and Cabinet officials.

Though Trump, Tillerson — and, by extension, White House Chief of Staff John Kelly and Defense Secretary James Mattis — have sought to allay suspicions of a rift between the president and his top diplomat, Trump seemed to complicate matters on Thursday night and again on Friday with vague hints referencing a "calm before the storm" while referring to his top military officials.