State Department officials confirm that Kim received permission to travel to New York, despite those sanctions, as has been done in the past for high-level diplomats. Pompeo and General Kim have met twice before, during Pompeo's recent visits to Pyongyang.

U.S. officials and outside experts see these talks as a critical step in determining whether to proceed with a summit between President Donald Trump and Kim Jong Un in Singapore on June 12.

General Kim was also seen at Kim Jong Un's side during last weekend's hastily convened mini-summit at the Demilitarized Zone with South Korea's President Moon Jae-In, a meeting sought by the North Koreans to salvage the summit after Trump's abrupt cancellation of the meeting in a letter two days earlier.

The New York meetings are taking place as a separate team of American experts has been meeting in North Korea to hammer out the outlines of a joint summit declaration focusing on the timing and components of what each side means by "denuclearization."

NBC News has been told by three U.S. officials that a recent CIA analysis concluded that Kim Jong Un would never give up the nuclear program his country has been developing for seven decades, a finding that conflicts with statements by Trump that Pyongyang intends to denuclearize in the future.

Separately, a team led by the foremost American scientific expert on the North Korean program, Stanford University Professor Siegfried Hecker, a former director of the Los Alamos weapons laboratory in New Mexico, reported this week that full denuclearization of North Korea's massive nuclear weapons and missile complex would take 15 years, and require an unprecedented inspection and verification challenge.

The short time frame to conclude pre–summit negotiations and logistics is further complicated by the changeover at the State Department: Pompeo, who was confirmed only last month to replace Rex Tillerson, has already sidelined Tillerson's top adviser on Asia, Acting Assistant Secretary of State Susan Thornton. Tillerson fought with White House hardliners to get Thornton nominated to that post.

Thornton, who always traveled with Tillerson, was noticeably missing from both of Pompeo's trips to North Korea, as well his New York meetings with Kim. In fact, Pompeo hinted at a shake-up during testimony to the Senate Foreign Relations Committee last week, saying he was "close to making a number of significant announcements about new members of the team, assistant secretary for East Asia affairs and South Asia among them."

There are reports that Thornton's nomination will be withdrawn after the Singapore summit. There is also a major vacancy in Seoul, South Korea: There's no confirmed U.S. ambassador, although Adm. Harry Harris, currently the head of the Pacific Command, has been nominated for that post.

U.S officials said Pompeo, who met with Trump before leaving for New York, will have dinner with General Kim on Wednesday night in New York in addition to meetings on Thursday.

Coordinating the multipronged summit is also challenging because of allied concerns: Japan's Prime Minister Shinzo Abe is seeking reassurance that potential normalization of relations between North and South Korea will not undermine his country's strategic interests. He is scheduled to meet with Trump in Washington next Thursday.