As one senior American intelligence official put it, “You’re not going to find the North Korean uranium project from these guys.” So the traditional methods of intelligence collection — using satellite imagery, phone and computer intercepts, and informants and agents of South Korea’s intelligence service — remain the main sources of information.

Still, the Web sites appear to have inflicted damage. North Korea’s spy agencies, which almost never admit to weaknesses, recently warned that South Korea’s “plot to overthrow our system, employing all manners and means of spying, is spreading from the periphery of our territory and deeply inland.” They vowed retaliation, especially against “human trash,” an apparent reference to the North Koreans who have betrayed their leaders’ code of silence out of principle or for pay to supplement their usually meager wages.

The informers’ networks are part of broader changes in intelligence gathering rooted in the North’s weaknesses. The first breakthrough came in the 1990s, when famine stoked by a breakdown in the socialist rationing system drove defectors out of the country and into the arms of South Korean and American intelligence agencies. The famine also led North Korea to allow traders to cross the border into China to bring home food, leaving them vulnerable to foreign agents, the news media and, most recently, the defectors and activists intent on forcing change in the North.

The first of their Web sites opened five years ago; there are now five. At least three of the sites receive some financing from the United States Congress through the National Endowment for Democracy.

The Web reports have been especially eye-opening for South Koreans, providing a rare glimpse of the aptly named Hermit Kingdom untainted by their own government’s biases, whether the anti-Communists who present the North in the worst light or liberals who gloss over bad news for fear of jeopardizing chances at détente.

“I take pride in my work,” said Mun Seong-hwi, a defector turned Web journalist with the site Daily NK, who works with the informers and uses an alias to protect relatives he left behind. “I help the outside world see North Korea as it is.”

Even in the days of the Iron Curtain, North Korea was one of the world’s most closed societies. There were few Western embassies where spies could pose as diplomats. And with citizens deputized to watch one another for suspicious activities, strangers could not escape notice for long.