The warning came as the communist country marked its withdrawal from a nuclear arms control accord a year ago with bellicose remarks towards Washington.

Two US non-government delegations visited the Yongbyon nuclear complex, 90 kilometers (50 miles) north of Pyongyang, where the communist regime said it was reprocessing weapons-grade plutonium.

"There have been comments out of Pyongyang and also out of Washington in terms of what we did or did not see in Yongbyon," said US congressional staffer Keith Luse who arrived here after a five-day trip to North Korea.

"It is simply premature and speculation for anyone to draw conclusions based on comment out of Pyongyang and Washington."

Luse and his colleague Frank Jannuzi lead one group while the other included former State Department official Jack Pritchard and two nuclear experts.

It was the first trip by outsiders to Yongbyon since UN inspectors were expelled a year ago.

North Korea said Saturday it showed "nuclear deterrent" to US delegates.

It did not say exactly what was shown, but The Washington Post said Sunday North Korea disclosed plutonium, an ingredient for a potential nuclear weapon, to the US delegations.

"One official said it appeared that the delegation had been shown what the North Koreans described as recently reprocessed plutonium," the daily said, quoting a US official who heard initial details of the trip.

"North Korean officials told the experts the material has not been placed in a nuclear device and that it was prepared to 'freeze' it to resolve the crisis over Pyongyang's nuclear ambitions," it said.

North Korea agreed in 1994 to mothball its Yongbyon complex but fired up the facilities after the latest nuclear crisis erupted in October 2002.

North Korea has claimed it completed reprocessing 8,000 spent fuel rods at Yongbyon, which could produce enough plutonium to make up to eight more nuclear bombs.

The arrival of Luse and Jannuzi here came as North Korea marked the first anniversary of its withdrawal from the non-proliferation treaty (NPT) with a pledge to step up war preparations.

The official Korean Central News Agency said North Korea "solemnly declared at home and abroad that it would keep strengthening self-defensive means to check the nuclear preemptive attack of the US.

"The world is now watching whether the US has a true will to settle the nuclear issue on the Korean peninsula on the principle of simultaneous actions and peaceful co-existence," it said.

Rodong Sinmun, the North's ruling communist party newspaper, said Sunday Pyongyang would step up military preparations against "a surprise war of aggression" by Washington.

"For the army and the people to get ready for a war is an important guarantee decisive of their victory and defeat in the war," Rodong said, attributing the fall of Iraqi leader Saddam Hussein to a lack of war readiness.

North Korea offered last week to refrain from producing and testing nuclear weapons in what it said was a "bold concession" to the United States.

Efforts to reconvene the nuclear crisis talks following the inconclusive first round in Beijing last August have so far failed amid differences over the scope of negotiations.

The North demands an agreement on "first-phase actions" including the lifting of sanctions against North Korea and a resumption of energy aid in return for the nuclear freeze.

Washington accused North Korea of setting preconditions, insisting Pyongyang must verifiably scrap its nuclear weapons.

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