The widely rumored nominee for that post, Victor Cha, was escorted through the West Wing several months ago to meet people, including senior staff, according to an official and a source familiar with the matter.
The administration sent Cha's name to Seoul in December, where he received swift approval, another source familiar with the matter said. That process -- almost always a quick, rubber-stamp affair -- only happens after candidates have received security clearance and gotten sign-off from the White House.
But nothing has happened since then, and there are questions -- and confusion -- about what will happen next. The hold-up is confounding, say observers, and potentially damaging to US security interests at a time when understanding nuances on the Korean Peninsula -- home to more than 28,000 US troops -- is critical.
"South Korea is important. You really need somebody monitoring it closely, and nobody will have the access an ambassador has," said Ronald E. Neumann, president of the American Academy of Diplomacy and a former ambassador to Algeria, Bahrain and Afghanistan. "You can't just keep tabs on where that's going and how it affects policy from Washington."
The White House and Cha did not return requests for comment for this story.
The situation in Seoul is part of a broader problem, as the Trump administration is leaving embassies across the world without chief envoys, including places that are central to American interests, such as Saudi Arabia, Germany, the European Union, Egypt and Jordan.
But there's little sign that vacancies in embassies across the world will be filled anytime soon.
The American Foreign Service Association, which represents active and retired diplomats, has calculated that in its first year, the Trump White House has nominated 31% fewer ambassadors than the Obama administration did in the same time period. The delay has created consternation and ruffled feathers in capitals abroad, so much so that some publications are now openly calling on the US Senate to confirm their ambassador.
Cha, a scholar and prize-winning author on Asian affairs, served as director for Asian Affairs on President George W. Bush's National Security Council. He has been critical of the Trump administration's decision to scrap the Trans-Pacific Partnership, which he's said would have yielded significant strategic benefits for the US, particularly in light of challenges from China.
Former White House chief strategist Stephen Bannon, along with allies who represented President Donald Trump's populist, protectionist campaign views, had previously targeted State Department nominees who they thought were too pro-trade or whose approaches to China weren't tough enough, in their estimation.
But Cha has also been an advocate for a muscular US presence in Asia, telling a Senate committee in April that China is "trying to erode US credibility, reliability and resiliency in the region, and replacing it with the fact that they are there, they are big, and they have a lot of money in their pocket."
In that April testimony to the Senate Armed Services Committee, he pointed to "a systematic plan by the North Koreans over the past decades to develop a capability that seeks to threaten the US homeland."
"I cannot think of a more proximate threat to our security at this point," Cha said.
Among South Korean officials, there's confusion about why Cha's nomination isn't moving forward given the stakes and tensions on the peninsula, a person familiar with the situation said. They're getting no explanations from the administration, sources tell CNN, and among themselves, think the problem must be internal US dysfunction.
At the State Department, spokeswoman Heather Nauert said the US embassy in Seoul has good leadership despite the lack of an ambassador.
"I can't get ahead of the White House and announce anything, but I can tell you we are in good hands," Nauert said Tuesday, pointing to the chargé d'affaires, the diplomat who heads an embassy in the absence of an ambassador.
"I know that the secretary and the White House are working hard to determine more people, more qualified candidates to take those positions," Nauert said.
Amid deep concerns across Washington about the State Department, there was widespread praise about news that the Trump administration could send a veteran foreign policy expert as ambassador to South Korea. As deputy head of the US delegation to multinational talks with North Korea that started in 2003, Cha has firsthand experience navigating the shoals of negotiations with Pyongyang.
The New York native and father of two sons would likely also be the only ambassador to have a film cameo as himself in a major Hollywood action film -- 2012's "Red Dawn," the tale of a Russian-backed North Korean invasion of the West Coast of the United States.
Cha now teaches at Georgetown University, runs the Asian studies program within its School of Foreign Service, and is a senior adviser at the Center for Strategic and International Studies.
"At a time when the value of expert knowledge is discounted, this appointment will stand out," Scott Snyder, director of the program on US-Korea Policy at the Council on Foreign Relations.
Analysts note that the US doesn't have the best possible visibility on developments at a time when the situation on the Korean Peninsula is especially fluid. North and South Korea are engaging in talks that were initially limited to the Olympics but have since expanded to military issues.
Beyond South Korea, there is concern about other ambassadorial posts remaining unfilled.
The German newspaper Bild tweeted an oped on the subject to lawmakers, calling on them to send nominee Richard Grenell sooner rather than later. "Dear US Senate, here's why Germany needs @richardgrenell as US ambassador in Berlin asap," the tweet read.
Politics have interfered with some nominations, as a few candidates met with objections from lawmakers, slowing votes on their nominations.
Grenell, a former spokesman to the UN for George W. Bush, ran into headwinds because of previous comments about women.
Democrats raised questions about Randy Evans, a Georgia lawyer nominated to be envoy to Luxembourg, because of his record on voting rights.
Lawmakers put a hold on the nominee for ambassador to Singapore. Former deputy national security adviser KT McFarland, a close ally of former national security adviser Michael Flynn, angered lawmakers, who thought she was less than transparent in answering questions about Trump campaign contacts with Russia.
The problem of empty ambassador posts is compounded by a steady exodus
of diplomats from the State Department, where morale has withered under Secretary of State Rex Tillerson's plan to cut the agency by as much as a third.
American diplomats are stepping in to serve in the absence of an ambassador in places across the globe, including Australia, Austria, Hungary, Sweden, Ireland, Belgium, Bolivia, Argentina, the Democratic People's Republican of Congo, South Africa, Qatar and Turkey, to name only some.
Neumann, the former ambassador, said that in those situations a chargé d'affaires will have a more difficult time getting the access that an ambassador would -- and that's especially so in countries with an acute awareness of rank and hierarchy.
"It's just harder," he said, "They don't get the same status that ambassador's do."