But Dr Siegfried Hecker suggested that US policymakers would be foolish to assume North Korea could not produce nuclear weapons.

Hecker, senior fellow of the Los Alamos National Laboratory in New Mexico, said during his first public testimony on his trip two weeks ago that North Korea also denied US claims that it confessed to a US claim it had a uranium enrichment program.

The US accusation sparked a nuclear crisis in October 2002 which has defied a drive to find a diplomatic solution.

Since then, North Korea has restarted a five megawatt nuclear reactor at its notorious Yongbyon complex and is piling up plutonium at the rate of six kilogrammes a year, Hecker told the Senate Foreign Relations Committee.

But a larger 50 megawatt reactor at the complex, frozen under a now ruptured 1994 anti-nuclear deal with the United States, was in a state of disrepair and clearly inoperable, he reported.

Hecker, part of two unofficial government delegations to North Korea, said he was shown what appeared to be a sample of reprocessed plutonium, a key ingredient for nuclear weapons.

North Korean officials described the substance as evidence they had a nuclear "deterrent."

"At Yongbyon they demonstrated they most likely had the capability to make plutonium metal," he said.

"However, I saw nothing and spoke to no one who could convince me that they could build a nuclear device with that metal, and that they could weaponize such a device into a delivery vehicle."

Later he told reporters: "It would be unreasonable to assume, and also just not smart to assume, they cannot make a rudimentary weapon,"

"All observations I was able to make are consistent with the sample being plutonium," he said, but stressed that without scientific instruments he could not assess whether the substance came from a recent reprocessing operation which North Korean officials said was completed by June last year.

Hecker told the Foreign Relations Committee, a day after testifying in closed session, that Pyongyang had been as good as its word in removing 8,000 spent fuel rods from a holding pond, where they had been kept under international observation until the crisis erupted.

"The spent fuel pond is empty. The approximately 8,000 fuel rods have been moved," he said.

The fuel rods were estimated to contain between 25 kg and 30 kgpounds) of plutonium. "We could not definitively substantiate that claim," said Hecker.

But he said laboratory staff had showed "the requisite facility, equipment and technical expertise and they appear to have the capacity" to extract the plutonium.

Two US delegations, which also included a former US policymaker, an academic and two congressional staffers, were also able to confirm that Pyongyang had restarted a small nuclear reactor at Yongbyon.

Pyongyang officials denied to the delegation US claims they had owned up to having an enriched uranium program during an October 2002 meeting with State Department Asia envoy James Kelly, Hecker said.

He quoted North Korean Vice Minister Kim Gye Gwan as telling him "'We have no program, we have no equipment, and we have no technical expertise for enriching uranium.

But the State Department Wednesday stuck to its guns.

"There were numerous officials at that meeting," said deputy spokesman Adam Ereli.

"What was said was vetted by a number of translators, there was no doubt in the minds of the officials who were in the meeting or in the translations that were made of the comments, and subsequently analyzed, about what was said and what was its import."

Hecker pointed out that a member of his delegation, Jack Pritchard, a former senior State Department official, took part in the Kelly meeting, and was sure he had heard clearly that the North Koreans had admitted to having a highly enriched uranium program.

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