A 1994 agreement between the United States and North Korea led to a freeze of the North's plutonium-based nuclear-weapons program. As part of the agreement, the United States promised to help North Korea build reactors that relied on nuclear technology that would be difficult to use for weapons. But not long after, North Korea began pursuing another path to produce nuclear weapons, acquiring technology from Pakistan to process highly enriched uranium.
The bilateral agreement, known as the 1994 Agreed Framework, froze North Korea's plutonium-based nuclear-weapons program, but did not lead to the country abandoning its pursuit of nuclear weapons.
One person who has intimate knowledge of Pyongyang's nuclear ambitions is Hwang Jang Yop, a high-ranking North Korean defector. He left the country in 1997 after serving as secretary of international affairs for the ruling Labor, or Communist, Party.
In a recent interview with VOA's Korean Service, Mr. Hwang said chief North Korean negotiator, First Vice Foreign Minister Kang Sok-Ju, told him Pyongyang intentionally drew out negotiations with the United States.
"When I asked the first vice foreign minister why we have dragged in negotiations with the United States, he said it is because we want to produce even one more weapon," he said.
Mr. Hwang says the party's secretary of military industry initially asked him to try to buy plutonium from the then-Soviet Union, to supplement what North Korea already had. But by the fall of 1996 the secretary told Mr. Hwang North Korea had resolved its so-called "nuclear problem," not through help from Russia or China, but by concluding a uranium-based nuclear-weapons agreement with Pakistan.
"I heard that a technical delegation from Pakistan visited North Korea," he said. "North Korea had researched the possibility of using enriched uranium, instead of plutonium, to produce nuclear weapons. The problem was resolved in one stroke by Pakistan. I heard indirectly that North Korea agreed to transfer missiles in exchange for an enriched-uranium program."
Mr. Hwang said he began hearing about a complex of caves at Kumchangri, about 160 kilometers north Pyongyang, which is where he believes the highly enriched uranium program was hidden.
The caves also aroused U.S. suspicions. The negotiator who signed the 1994 Agreed Framework for the U.S. side, Robert Gallucci, points to two American inspections of the caves at Kumchangri in 1999.
"There was a thought that the cave, the cavern, might contain either a centrifuge operation," he said. "It could have contained a small nuclear reactor. It could have contained a small chemical separation or reprocessing facility. In other words, some nuclear-related activity. It could have been a storage place for separated plutonium or for nuclear weapons. I do not know that the United States government came to a view on what was to be there. But it was certainly a very well-secured site by the North Koreans. And there was reason to believe it was somehow related to nuclear activity of some kind."
Mr. Gallucci says Washington was able to make the visits only because the two sides were talking under the 1994 agreement, but that inspectors found nothing to corroborate the suspicion that Kumchangri was a secret nuclear site.
But U.S. concerns over a North Korean uranium-based nuclear weapons program have since been confirmed by Pakistani nuclear-scientist Abdul Qadeer Khan, who recently admitted that his international proliferation ring transferred uranium-enrichment equipment and technology to North Korea.
"I think the United States probably has extremely good evidence that the North Koreans acquired centrifuge components and technology from Pakistan," said Mr. Gallucci. "We know that, I think, from our own sources. But also, we all should be aware that A.Q. Khan, the Pakistan father of the enrichment program and sometimes called the father of the bomb in Pakistan, has admitted transferring centrifuge technology, selling it, to North Korea. So, I do not think there is really much question about this. I do not know why the North Koreans insist on refusing to admit this."
Chief U.S. negotiator James Kelly first confronted North Korea with this knowledge in October 2002, before A.Q. Khan's confessions. American officials say North Korea acknowledged having a highly enriched uranium program then, although Pyongyang subsequently denied it.
Mr. Gallucci says there is no question that North Korea has the technology. What is not so clear, he adds, is whether the program is producing nuclear weapons.
"There is a big difference between having rotors, end-caps, inverters and a lot of the components that go to make up a gas centrifuge machine, on the one hand, and having those components put together, not only in a machine, but thousands of machines piped together, running effectively, enriching uranium," he said. "I do not know that North Korea has accomplished all that. I do not know that it has not, but I do not know that it has. I think it has sufficient components to construct a centrifuge cascade, which, if they were to accomplish this, would produce highly enriched uranium in a matter of some few years."
Mr. Gallucci says he is not sure how far along North Korea is in assembling or operating the gas centrifuge cascade it needs for uranium reprocessing. He estimates that Pyongyang could produce enough uranium for at least one nuclear weapon, in about three to five years.
This uranium program, whatever its development phase, is in addition to the North's plutonium program, which international experts say may have already produced five or six nuclear weapons.