The new declaration set off a scramble in American intelligence agencies — under fire for their assessment of Iraq's nuclear capability — to determine if the North Korean government of Kim Jong Il was bluffing or had succeeded in producing the material undetected.

Officials said today that the answer was unclear. A preliminary set of atmospheric tests for the presence of a gas given off as nuclear waste is reprocessed into plutonium is the best indicator the United States has from one of the world's most closed nations. The most recent tests suggested that nuclear work has accelerated, but the results were inconclusive. More test results are expected at the end of this week.

"It's the mirror image of the Iraq problem," one official said. "We spent years looking for evidence Iraq was lying when it said it didn't have a nuclear program. Now North Korea says it's about to go nuclear, and everyone is trying to figure out whether they've finally done it, or if it's the big lie."

North Korea boasted in April that it was working to convert its 8,000 spent nuclear fuel rods into weapons-grade plutonium. The rods had been held under seal by international inspectors until the inspectors were expelled from the country on Dec. 31. Several months ago, American spy satellites saw the rods being hauled away from a storage shed, though it is unclear where they were taken.

North Korea's latest declaration, if true, would pose a direct challenge to President Bush, who said two months ago that a nuclear-armed North Korea "will not be tolerated." 

Mr. Bush will be faced with difficult choices. Early this year, he decided it was too risky to take military action against the the North's main nuclear reprocessing plant, at Yongbyon, even before the reprocessing started. Now, though, the Pentagon may be asked to revisit the military options that Mr. Bush has always said are a last resort.

But the president must also decide whether to negotiate with the North — under its implicit nuclear threat — or hold fast to his insistence that any talks must include other regional nations, and that nuclear blackmail would be met with increasingly harsh sanctions.

In the months since the spent nuclear fuel rods were transported to an unknown location, North Korea has regularly escalated its claims. First, it said it needed a "strong physical deterrent" to protect itself against invasion by the United States. Then, after the Iraq war, it said it needed a "nuclear deterrent." 

But intelligence agencies have scant evidence that North Korea has produced enough plutonium to build a nuclear weapon, officials said. As recently as two weeks ago, American intelligence officials told South Korea and Japan that they believed that, at most, only a few hundred rods had been converted into weapons-usable material. Then they warned that the North was experimenting with the conventional explosives needed to ignite a nuclear explosion — further evidence of intent to produce weapons.

The C.I.A. believes that North Korea may have produced two nuclear weapons in the early 1990's, but the evidence is in dispute. In any event, officials say the ability to produce a half-dozen more would greatly increase the North's leverage: it could conduct a nuclear test, store a few weapons and threaten to sell any leftover plutonium.

The North's latest declaration came on Tuesday in New York, during an unannounced meeting between North Korean diplomats at the United Nations and Jack Pritchard, a State Department official who handles North Korea issues.

"They went into new territory," said one official familiar with the meeting. The North Korean diplomats read a statement from Pyongyang declaring that the reprocessing of the rods, a chemical process that the North perfected in the late 1980's after receiving considerable foreign help, had been completed on June 30. 

The North Koreans then said weapons production was beginning. "They didn't say how long it would take, and they didn't threaten to sell anything," a senior official said. 

The State Department spokesman, Richard A. Boucher, said today that "North Korea has made a variety of claims" in the past, some false.

"We've always said that we will look at all of the available information, not just what they happen to claim or say at any given moment," he said. 

Despite the effort to play down the news — Mr. Bush's aides have refused to call the Korea situation a "crisis," fearing that would play into Mr. Kim's strategy — there is a debate in the administration about North Korea's intentions.

Some see last week's declaration as a negotiating ploy. They believe that North Korea has been frustrated by Mr. Bush's refusal to engage in one-on-one negotiations, insisting instead that China, Japan and South Korea act as partners in finding a regional solution. Mr. Bush's real motivation for resisting bilateral talks, his aides say, is that he fears that Asian nations will press the United States to reach some kind of deal similar to the one the Clinton administration signed — a "freeze" on nuclear activity in return for aid.

Other officials believe that Mr. Kim's government has simply decided that it can make both Washington and its Asian neighbors accept North Korea as a new nuclear power.

"There's a body of thought that they are just getting everybody accustomed to the idea," a senior administration official said. "So when they say one day, `We've gone nuclear,' it's no shock."

China Sends Letter to Kim SEOUL, South Korea, Tuesday, July 15 (Reuters) — President Hu Jintao of China has sent a letter to Mr. Kim, the North Korean leader, and South Korea said today that it hoped that the message would help persuade Pyongyang to agree to multilateral talks on its nuclear aims. 

KCNA, the North's official news agency, said China's deputy foreign minister, Dai Bingguo, met Mr. Kim on Monday. KCNA did not disclose the substance of the letter.