To officials and analysts in South Korea, Mr. Kim’s decision to shut down his country’s only known nuclear test site, in Punggye-ri in northeastern North Korea, and his moratorium on long-range missile tests, are some of the “trust-building steps” that they have hoped Mr. Kim would take to help improve the mood for dialogue in Washington.

Mr. Kim spent last year conducting a series of nuclear and intercontinental ballistic missile tests, raising tensions and a risk of war with the United States. But he has initiated a dramatic about-face since January with a sequence of diplomatic maneuvers, including a summit meeting with President Xi Jinping of China in Beijing last month in his first trip abroad as leader, and his invitations to Mr. Moon and Mr. Trump for summit talks.

Analysts in the region are deeply divided over Mr. Kim’s motives. Some argue that Mr. Kim just wanted to use negotiations to buy time and ease international sanctions, never intending to abandon his nuclear weapons. But others say that Mr. Kim would eventually give up his nuclear arsenal if he were provided with the right incentives, such as security guarantees, like a peace treaty and normalized ties with Washington, and the economic aid he needs to rebuild his economy.

His latest announcement came one day after North and South Korea installed what officials said was the first-ever hotline between their top leaders, another sign of improving relations on the divided Korean Peninsula.

Mr. Moon was expected to use the hotline, which was installed in his office, to talk with Mr. Kim before the two leaders hold their summit meeting on the Korean border next Friday. But no date has been set for their first call.

The two Koreas have run a telephone hotline at the so-called truce village of Panmunjom — the site for the inter-Korean meeting — for years. Duty officers from both sides man their telephones at Panmunjom daily in case one side calls the other. The line has been cut off at times when bilateral relations have soured, but communications there have been restored.

But the two countries have never run a direct hotline linking their top leaders’ offices, officials said.