BEIJING -- The head of the International Atomic Energy Agency said yesterday that a mysterious building in North Korea, if outfitted with additional equipment, could function as a plutonium reprocessing center, the core of a nuclear weapons program.
Hans Blix, the director general of the agency, just returned from a six-day visit to North Korea. He is the first Westerner known to have visited the sprawling laboratory in Yongbyon, 60 miles north of the capital, Pyongyang.
The laboratory is the focus of suspicions about North Korea's nuclear intentions.
The observations of Mr. Blix, who also saw three nuclear power plants and a surprising network of subterranean tunnels, provide the firmest evidence yet that North Korea may have tried to build a nuclear bomb, or perhaps is still trying. He said his inspection would be followed within a few weeks by a team that will examine all of North Korea's declared nuclear sites.
Mr. Blix was very careful to describe only what he saw and not to accuse North Korea of trying to develop nuclear weapons.
"I don't like very much to speculate," he said at a news conference in Beijing upon his return.
The visit by Mr. Blix and his advisers, however, seemed to raise as many questions as it answered about whether North Korea was trying to develop nuclear weapons and how far it might have progressed.
North Korea told the agency that it had extracted plutonium for research purposes in the "radio chemical laboratory," as it refers to the Yongbyon building. Mr. Blix said that the laboratory, built beginning in 1987, is about 196 yards long and several stories high, about the size of two football fields.
"It is termed a laboratory, and of course they have used it for testing," he said. "If it were in operation and complete, then it would certainly in our terminology be called a reprocessing plant."
Mr. Blix said he was told that North Korea had built the complex entirely on its own. He added that North Korean nuclear scientists, some of whom he described as very knowledgeable, were probably trained in their own country and in the Soviet Union.
Mr. Blix said that the Yongbyon plant was incomplete and that there were "several pieces missing."
He was told that the construction was 80 percent complete and that 40 percent of the equipment was installed. The North Korean authorities told him that the rest of the equipment "was on order but not yet delivered." It was not clear when it would arrive.
The fact that the building was partly empty and not in operation during Mr. Blix's visit raised the possibility that some plutonium-processing equipment had been removed specifically for the visit or that North Korea had failed in its attempt to build a complete reprocessing plant.