The timing of Mr. Pence’s comments was significant, coming just before Pyongyang’s lead negotiator over the nuclear program, Kim Yong-chol, is expected to arrive in Washington to meet on Friday with Secretary of State Mike Pompeo. The North Korean negotiator is usually not permitted to travel more than a short distance from the United Nations headquarters in New York, where Mr. Pompeo met him last year, so the invitation to Washington was seen as a gesture to make arrangements for the summit meeting.

Mr. Kim, the negotiator, is a former general and intelligence chief believed to be about 74, and is viewed as a member of the inner circle of North Korean leaders. He is also widely regarded as the architect of an attack on a South Korean ship in 2010 that killed 46 South Korean sailors.

But so far, his meetings with Mr. Pompeo, in both Pyongyang and New York, have yielded few results.

While the United States has demanded that North Korea turn over its nuclear weapons before it sees any significant sanctions relief, the North has argued for a step-by-step approach — including that the United States withdraw troops and weapons from the Korean Peninsula.

Breaking that logjam will be the key to the meeting with Mr. Pompeo, American officials said. North Korea has still not taken the first step demanded by the United States: Providing an inventory of its nuclear weapons, its stockpiles of nuclear material, its production facilities, missile fabrication plans and launch sites.

The North has instead said that it would give Washington a target list. Mr. Trump’s negotiators have argued that they already have a list, but want evidence that the North was being truthful in its declarations.