Undersecretary of State John Bolton, meeting with Japanese officials in Tokyo, warned Wednesday that North Korea's denial it has a nuclear program based on uranium could hurt efforts to resolve the crisis.

"I think North Korea's unwillingness to discuss the uranium enrichment program could subvert President Bush's determination for a peaceful, diplomatic resolution of the North Korean issue," Bolton said in an interview with Japanese public broadcaster NHK.

The questions about North Korea's nuclear capabilities are expected to overshadow the six-party talks in Beijing that begin next Wednesday with China, Japan, Russia and South Korea also taking part.

At issue is whether North Korea has only a plutonium-based nuclear program, as it claims, or whether Pyongyang also has a uranium-based program, as the United States maintains.

There's also uncertainty about whether North Korea has made nuclear weapons and whether they can mount them on a missile and fire them.

The plutonium program is believed to be more of an immediate threat than the alleged uranium one, which does not require large-scale, easily detectable facilities and could require at least several years of operation before it can produce a bomb.

U.S. officials believe North Korea has at least one or two nuclear bombs from plutonium, though some experts believe Pyongyang does not have the technology and resources to mount a nuclear warhead on a missile.

The recent confession of Abdul Qadeer Khan, Pakistan's proliferator of nuclear secrets, suggests North Korea's uranium program "is of longer duration and more advanced than we had assessed," U.S. Assistant Secretary of State James Kelly said last week in Washington.

North Korea denies receiving nuclear secrets from Pakistan.

"There is no agreed estimate of anything," said Leon Sigal, a North Korea expert. "As with Iraq, there is significant disagreement in the intelligence community about pieces of this."

North Korea will likely try to capitalize on the uncertainty, brandishing the threat of what it vaguely describes as its "nuclear deterrent" in an effort to extract concessions.

U.S. negotiators will likely hold firm, demanding that North Korea dismantle all nuclear projects in a verifiable, irreversible way.

A resolution is possible if the two adversaries move toward a step-by-step process under which North Korea - perhaps the most secretive country in the world - allows unprecedented access to its most guarded sites, and the United States and its allies provide sweeping security assurances and economic aid.

Pyongyang might pursue two tracks: offer to freeze activities at its plutonium-based site at Yongbyon in exchange for concessions, and persist in denying or downplaying claims it has a program based on uranium enrichment. It could also attack the credibility of U.S. intelligence following the failure to find weapons of mass destruction in Iraq.

North Korea might feel emboldened by reports that China, a traditional ally of Pyongyang that is wary of U.S. influence in Asia, has not accepted the U.S. contention that the North has a uranium-based weapons program. However, China wants the Korean Peninsula to be free of nu clear weapons and could pressure the North to curtail its belligerence.

On Thursday, South Korea's Yonhap news agency reported that North Korea had conveyed its willingness to discuss the dispute over its alleged uranium-based program with Washington.

"We understand that North Korea recently told the government of a third country that it is willing to discuss the HEU issue with the United States," Yonhap quoted a high-ranking government source as saying. HEU stands for highly enriched uranium issue.

The official said North Korea did not admit having a highly enriched uranium, program. Officials at the South Korea's Foreign Ministry and presidential office were unable to confirm the Yonhap report.

Some security analysts believe the mystery will put pressure on the United States to be more explicit about what it knows.

"Unless the U.S. introduces a high-level defector with certain knowledge of the North Korean (uranium program) locations, or can send the IAEA or other inspectors to the right place, U.S. intelligence credibility will not be reinforced," said Larry Wortzel, a former U.S. military attache in Beijing and now an analyst at the Heritage Foundation in Washington.

North Korea expelled inspectors of the International Atomic Energy Agency, or IAEA, from frozen nuclear facilities at Yongbyon after U.S. officials alleged that the North admitted it had a uranium-based program in late 2002. North Korea then restarted the Yongbyon site as Pyongyang and Washington traded accusations that each side had failed to honor a 1994 nuclear deal.

In a rare, recent show-and-tell by North Korean scientists at Yongbyon, an American was allowed to pick up - with a gloved hand - a glass jar that allegedly contained 200 grams of plutonium metal, a key ingredient in nuclear weapons. The jar was heavy and slightly warm, and tests showed the substance was radioactive.

But the American, Siegfried Hecker of the Los Alamos National Laboratory, and other U.S. scientists couldn't confirm what was in the jar. They concluded 8,000 spent fuel rods containing plutonium had been removed from a fuel pond, but they could not substantiate North Korean claim the rods had been reprocessed to extract the metal.

"I saw nothing and talked to no one that allowed me to assess whether or not they have the ability to design a nuclear device," Hecker said last month.

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