Yun has led the US diplomatic outreach to North Korea since taking his post under former US President Barack Obama in 2016.

Yun travelled to North Korea last June to help secure the release of American student Otto Warmbier, whose subsequent death further soured relations between Washington and Pyongyang.

US President Donald Trump has charted a more confrontational approach to North Korea and its nuclear weapons programme, forcing Yun to often operate under mixed White House messages of impending military strikes as well as diplomatic overtures.

"Ambassador Joe Yun, a respected member of the Senior Foreign Service, has decided to retire for personal reasons," State Department spokeswoman Heather Nauert said in a statement.

Secretary of State Rex Tillerson "has reluctantly accepted his decision and wished him ?well," she added.

Yun told CNN "it was completely my decision to retire at this time.