State Department spokeswoman Heather Nauert said Yun decided to leave for personal reasons and Secretary of State Rex Tillerson had no choice but to accept his resignation.

The decision comes just as Pyongyang has said it is willing to talk directly to the U.S.

Joseph Yun, the chief U.S. nuclear negotiator and proponent of dialogue with North Korea in the Trump administration, is resigning at the end of the week.

Yun told CNN, "It was completely my decision to retire at this time." He also told the Korean Embassy in Washington that he was "retiring," not "resigning."

But there are suspicions that Yun stepped down because Washington is taking an increasingly belligerent stance towards Pyongyang.

In November last year, Reuters quoted a White House official as saying, with a note of sarcasm, "He's such a dreamer." Reuters said Yun "hopes his diplomatic efforts can lower the temperature in a dangerous nuclear stand-off. Most were deeply skeptical about his chances."

Yun told a media outlet it is time for a close aide who is on the same page as U.S. President Donald Trump to take charge of North Korea policy. That suggests he was forced out under direct pressure from the White House.

It was Yun who revived the so-called "New York channel" of informal dialogue with North Korea through its permanent mission at the UN headquarters in the city.

In June last year, he flew to Pyongyang to negotiate the release of Otto Warmbier, an American college student who had been detained there for stealing a propaganda poster. Warmbier was then already in a coma and died days after his return.

