SEOUL, South Korea >> If North Korea’s fifth nuclear test last Friday rattled outside policymakers by demonstrating technological advances in the country’s weapons program, it also reminded them of how difficult it remains to parse the country, one of the world’s most isolated and secretive.

Not even those of us in South Korea saw what was coming that Friday morning until European monitors of seismological signals reported a tremor emanating from the Punggye-ri test site. South Korea’s president, Park Geun-hye, had to cut short her state visit to Laos. The prime minister and the unification minister had to rush back to Seoul from trips to provincial cities.

For months, the Defense Ministry here has given the standard — and the safest — answer when asked about the likelihood of another nuclear test by the North: The country was ready to conduct one at any time, whenever its leader, Kim Jong Un, gave the go-ahead.

That answer, though, is hardly satisfying for those of us who cover the region — and who live within striking range of North Korean missiles, which Kim seeks to mount with nuclear warheads.

Still, it is often a fruitless exercise to try to predict North Korean developments. Being on constant alert, always ready for surprise, is the safest course.

I remember how startled I was when North Korea announced the death of Kim’s father and predecessor, Kim Jong Il, in 2011. And The Times was not the only organization caught off guard: Outside intelligence agencies did not know about the leader’s death until the North announced it two days after the fact. Many journalists and senior government officials, including presidential aides, were out for lunch when an announcer appeared on North Korean television and read the news in a weepy voice.

The news of the North’s latest nuclear test was less dramatic but still took many people — including this reporter — by surprise.

For decades, U.S. spy satellites have been scouring key North Korean military sites, including the hilly Punggye-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 NIS, 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. (NIS 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 NIS 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 NIS, 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.

In recent months, it took the unusual steps of announcing high-profile defections from the North and the executions of top officials there, invariably citing them to portray North Korea as unstable and desperate under Kim.

Andray Abrahamian, a North Korea expert who works for the Choson Exchange, recently warned of unverified rumors about North Korea finding audiences via the foreign news media.

“North Korea’s opacity makes it seemingly easy to start rumors about what may be taking place there, as corroboration often seems too difficult to pursue,” Abrahamian wrote. “Reader interest in North Korea — and especially in salacious news — is high, making it very hard for journalists and editors to resist repeating a rumor when they are far from the story and thus less accountable for it.”

What does all this mean for a Times bureau chief stationed in Seoul? How does it affect my reporting?

One of the first things I do when I see a dramatic piece of news spreading through the media is to trace it to its origin. Very often, it starts with a report in the South Korean news media based on a single anonymous source, and I try to gauge the news outlet’s track record and potential ideological bias.

It remains shocking how one such report or rumor can mutate, distorted and amplified, as it is repackaged by other news outlets abroad. (One recent example is the infamous “Kim Jong Un fed his uncle to dogs” story.)

Given the uncertainty and sensitivity surrounding North Korea, officials here habitually fall back on anonymity when talking to reporters. This, in turn, means that anonymously sourced stories abound in South Korean news media, especially when the report concerns North Korea. Living up to The Times’ strict guidelines on using anonymous sources means extra efforts to reach out to and crosscheck sources, always trying to find ones willing to go on the record — and, very often, letting stories go unpublished. (It often takes weeks and even months before such a story is, if ever, independently confirmed or disputed. By then, it has been long forgotten.)

The truth is that there is no foolproof way to report on North Korea. But it helps to remember that the two governments on the Korean Peninsula have been locked in a military standoff for decades, and that the North is ruled by a young, ambitious, cultlike leader obsessed with securing his — and his regime’s — survival by building a nuclear arsenal.

That helps explain what I consider a remarkable consistency in the North’s behavior and pronouncements when considered in the long term.

But it doesn’t help me, or anyone else, predict tomorrow’s North Korean news.

© 2016 The New York Times Company