WASHINGTON — The goal of a diplomatic meeting set for Thursday seemed simple: Nail down a plan for a second summit between President Trump and the North Korean leader, Kim Jong-un.

But finalizing the meeting itself — between Mike Pompeo, the secretary of state, and one of Mr. Kim’s top compatriots — proved harder than expected. The State Department announced early Wednesday that it had been canceled.

“Ongoing conversations continue to take place,” the statement said tersely.

The cancellation leaves little doubt that the diplomatic process between the United States and North Korea is now mired in quicksand after peaking in Singapore with the initial summit between the two leaders. There are mismatched demands and expectations on both sides, and the pitfalls have only gotten more obvious in recent weeks.

At the White House later Wednesday, Mr. Trump said the meeting would be rescheduled and insisted that “we’re very happy with how it’s going with North Korea.”