-- The U.S.'s top diplomat for North Korea is retiring just as the prospect of talks with North Korea seemed possible.
Ambassador Joseph Yun, the special representative for North Korea Policy, is retiring this week after more than 30 years in the Foreign Service.
Yun is yet another member of the Senior Foreign Service who is leaving while the department is still under a hiring freeze and many top roles have not been filled.
There has been growing frustration among the diplomatic ranks over the Trump administration's handling of foreign policy and Secretary of State Rex Tillerson's "redesign" plan of the department. Spokesperson Heather Nauert said in a statement Yun was retiring for "personal reasons and the Secretary has reluctantly accepted his decision and wished him well."
"We are sorry to see him retire, but our diplomatic efforts regarding North Korea will continue based on our maximum pressure campaign to isolate the DPRK until it agrees to begin credible talks toward a denuclearized Korean peninsula," the statement added.
Nauert confirmed today that the administration will keep Yun's position and seek a replacement, but she downplayed the impact of Yun's departure.
"The State Department has 75,000 people that work for us around the world. To imply that Ambassador Yun is the only one capable of handling North Korea would simply be wrong. We have a deep bench of very experienced people," Nauert said during a department briefing later today.
"We look forward to having Susan Thornton and also our other colleagues fill in on the gap," she added.
Thornton is the acting assistant secretary of State for East Asian and Pacific Affairs, who has been nominated for Yun's position.
But the top Democrat on the Senate Foreign Relations Committee blasted the administration for the departure, calling it "yet another setback in the Trump administration's chaotic and uneven approach to diplomacy in the Korean peninsula and to the overall institutional integrity of the State Department."
"Without the full weight of a robust diplomatic presence in the Asia-Pacific region, the United States will be left on the sidelines at a time when we can least afford it. The stakes are too high," Sen. Bob Menendez, D-N.J., said in a statement.
Yun was asked to stay on by the State Department for at least a few more months, but he declined, a source told ABC News.
There is currently no U.S. ambassador to South Korea, with the leading contender Victor Cha now out of the running, according to a senior administration official. Charge d'Affaires Mark Knapper leads the embassy in Seoul.
Yun was pivotal in the on-again, off-again back-channel talks with North Korea in recent years. The North has expressed a willingness to engage with the U.S., according to South Korean President Moon Jae-in's office.
Earlier this year the State Department's third-highest ranking official, Tom Shannon, the under-secretary of state for political affairs, announced that he was retiring. Of the five other under-secretary roles, only one is filled by a Trump administration nominee, and the remaining four are vacant, with two nominees awaiting confirmation.
Yun is known more recently for securing the release of college student Otto Warmbier last spring. After secret talks with high-level North Korean officials, Yun visited Pyongyang in June and brought Warmbier, who had spent months in a coma in North Korean custody, home.
Yun was also able to visit with the three other Americans held by North Korea -- the last known time they were seen.