For decades, American spy satellites have been scouring key North Korean military sites, including the hilly Punggy-ri site where the North has conducted all its previous underground nuclear tests. In recent years, private think tanks have also scrutinized the site, relying on commercial satellite imagery. The results, however, are often inconclusive.

Some outside news outlets, such as The Associated Press and the Japanese news agency Kyodo, operate bureaus in Pyongyang, the North Korean capital. But their reporters are not allowed to meet people or to travel freely.

Those trying to report on North Korea from the outside sometimes talk to sources — often paid sources — inside the North, but the accounts from such sources usually cannot be verified. Outside reporters also often rely on defectors from the country, but few defectors arrive with access to valuable intelligence on its nuclear programs or top leaders.

And, of course, all of North Korea’s news media is state-controlled, which makes it difficult to separate fact from propaganda.

South Korea’s National Intelligence Service, or N.I.S., is a frequent source of North Korean news in the South Korean media — which, in turn, is often eagerly picked up and repackaged throughout the international news media, feeding a high demand for updates. (N.I.S. often leaks information to several representatives of the local media, insisting that it be attributed to an anonymous source. The next day, the local media outlets offer identical reports — and the N.I.S. spokesman’s office refuses to confirm the information when reporters from foreign news organization call in.)

But analysts warn that the agency’s lack of political neutrality often taints its information.

The government of South Korea, especially N.I.S., has been accused of leaking selected information — or even incomplete and unverified intelligence — about the North to help influence domestic opinion and push its policies.