Yun told CNN, "It was completely my decision to retire at this time".
The ambassador post in South Korea has been vacant for the past year, adding to questions about the direction of United States policy as Trump has vacillated between talk of war and willingness to speak directly with leader Kim Jong-un.
Kang was addressing the UN Human Rights Council a day after the Olympic Winter Games closed in her country.
The top USA diplomat on North Korea policy said Tuesday he will retire this week, further depleting an already short-handed State Department even as Washington and Pyongyang inch closer to dialogue. "In the meantime, the United States and the world must continue to make clear that North Korea's nuclear and missile programs are a dead end", White House Press Secretary Sarah Huckabee Sanders said in a statement.
A senior State Department official told Reuters late previous year that Yun had sought direct diplomacy with North Korean officials at the United Nations in the hope of lowering the temperature in U.S.
Several US officials have downplayed previous suggestions that the administration is considering the use of a so-called "bloody-nose strike" - a military option that would be significant enough to force North Korea to question its nuclear ambitions but limited in scale as to avoid retaliation. "I think we need some serious diplomacy on this". "They stopped nuclear and missile tests". "And the USA is willing to talk to the North Koreans if they are prepared to put nuclear weapons on the table".
"If talks are possible only under the right conditions just as Trump said, the United States should create those conditions", Choson Sinbo said in its article.
The White House says it hopes pressure being applied by sanctions on North Korea will lead to a diplomatic path to solving the stalemate - but military action isn't off the table. But the U.S. has pressed on with ever tougher sanctions, including a new set on the North's shipping capabilities.
CBS quoted Yun as saying there were no policy differences "per se", but officials he has dealt with in South Korea told Reuters he had appeared increasingly frustrated with conflicting views within the administration on how to deal with the crisis.
Trump announced on Friday what he called the "largest ever" set of sanctions against North Korea and threatened a "phase two" if the measures aren't effective.
The February 23, 2018 global shipping advisory released alongside the sanctions designations flags what it says are "significant sanctions" risks to people who enable shipments of goods to and from North Korea, including related businesses, such as insurers, flag registries, shipping companies and financial institutions. They block assets held by the companies in the United States and prohibit USA citizens from interacting with them.
"They want to talk".
"Otherwise", he said, "we're not talking". "And we'll see what happens". "The messaging has been solid".