The best way to force change in the isolated North, he continued, is to disseminate outside information there to help ordinary citizens eventually rebel. South Korean TV dramas and movies smuggled from China are already popular in the North, he said.

Another sign of Mr. Kim’s weakening control, Mr. Thae said, is evident at the unofficial markets in North Korea where women trade goods, mostly smuggled from China. The vendors used to be called “grasshoppers” because they would pack and flee whenever they saw the police approaching. Now, they are called “ticks” because they refuse to budge, demanding a right to make a living, Mr. Thae said.

Such resistance, even if small in scale, is unprecedented, he added.

The spread of outside news and market activities could eventually doom Mr. Kim because his government “can be held in place and maintained only by idolizing Kim Jong-un like a god,” Mr. Thae said. “If he tries to introduce a market-oriented economy to North Korean society, then there will be no place for Kim Jong-un in North Korea, and he knows that.”

But the leader’s efforts to clamp down on information and products from outside North Korea have been unsuccessful because the police accept bribes in exchange for freeing smugglers and people caught watching banned movies and dramas.

“Kim Jong-un’s days are numbered,” Mr. Thae said on Wednesday.

After months of debriefing by the authorities in South Korea, Mr. Thae used meetings with the country’s politicians and the news media to suggest that North Korea was determined to be recognized as a nuclear power, just as India and Pakistan are.

Last year, the North conducted two nuclear tests and launched more than 20 ballistic missiles, and it has openly vowed to develop the ability to hit the United States with a nuclear warhead.

“It won’t happen,” Donald J. Trump, then president-elect, said at the time. During the election campaign, Mr. Trump had said he was willing to sit down with Mr. Kim and perhaps have a hamburger with him.