Bush committed his government to seeking a diplomatic rather than military solution to the standoff, while at the same time boosting defense cooperation with Asian allies on the front line against the erratic communist regime.

He added it had yet to be confirmed that Monday's blast, announced by Pyongyang, was in fact a nuclear detonation.

"But this claim itself constitutes a threat to international peace and stability," he said.

"We are working with partners in the region and in the United Nations Security Council to ensure there are serious repercussions for the regime in Pyongyang" as a result of the test, Bush said.

The chance of sanctions grew after the North's main ally China said it would support punitive action.

"If the US continues to harass and put pressure on us, we will regard this as a declaration of war and will take a series of physical countermeasures," said a foreign ministry statement carried by the North's official Korean Central News Agency.

It did not elaborate on the measures, but insisted it was still ready for talks to improve security and stability on the Korean peninsula. "We are ready for both dialogue and confrontation."

Bush said he had spoken with the leaders of the four other governments leading efforts to halt North Korea's nuclear effort -- Japan, China, South Korea and Russia -- and had found unanimous agreement on the need for "a strong Security Council resolution that will require North Korea to abide by its international commitments to dismantle its nuclear programs".

He said the resolution, being debated Wednesday at UN headquarters in New York, "should specify a series of measures to prevent North Korea from exporting nuclear or missile technologies."

Washington also wants sanctions that would prevent "financial transactions or asset transfers that would help North Korea develop its nuclear missile capabilities," he said.

The Security Council meeting would follow private talks Wednesday morning among envoys of the Council's five veto-wielding members -- Britain, China, France, Russia and the United States -- and Japan on harsh sanctions against Pyongyang, including inspection of all seaborne cargo to and from North Korea as well as financial restrictions.

UN chief Kofi Annan on Wednesday urged North Korea not "to escalate the situation any further" in reference to rumors that Pyongyang was planning a second nuclear test.

North Korea's message was reinforced by Kim Yong-Nam, who as head of the North Korean Supreme People's Assembly is effectively the regime's number two.

"If the United States continues to take a hostile attitude and apply pressure on us in various forms, we will have no choice but to take physical steps to deal with that," he said in an interview with Japan's Kyodo News.

He added: "The issue of future nuclear tests is linked to US policy toward our country."

Japan meanwhile ramped up its bilateral sanctions on North Korea, slapping a complete ban on imports and shipping and barring almost all the communist country's nationals.

"Considering the improving capability of North Korea's missiles and its nuclear capability, Japan is the country that is most affected by the actions of North Korea in terms of security," said Prime Minister Shinzo Abe.

It was not certain if China and Russia -- which both have veto power, tend to oppose international sanctions and have close ties with Pyongyang -- would back harsh sanctions.

"I think there have to be some punitive actions but also these actions have to be appropriate," China's UN ambassador, Wang Guangya, told reporters.

In North Korea itself little has emerged of the atmosphere since Monday's announcement, which was played down by state media, according to some of the few foreigners allowed to live in the hermit nation.

"It really has been a bit quiet," said one foreigner working for a UN aid organization.

North Korea has repeatedly insisted its nuclear programme is essential to deterring an attack from the United States.

At six-nation talks in September 2005 it appeared to have agreed to abandon its nuclear programme in exchange for energy and security guarantees, in what was seen as a major breakthrough.

But the North gave up and began boycotting the talks just two months later after the United States imposed its own sanctions on a Macau bank it said was laundering money for the Pyongyang regime.

earlier related report
NKorea warns of more nuclear tests if US keeps up pressure
Seoul (AFP) Oct 11 - North Korea Wednesday warned of more nuclear tests if the United States keeps pressuring it as South Korea urged the UN Security Council to send a "clear and firm" message to the communist state.

"If the US continues to harass and put pressure on us, we will regard this as a declaration of war and will take a series of physical countermeasures," said a foreign ministry statement carried by the official (North) Korean Central News Agency.

The ministry said its statement was in response to US attempts to secure tough sanctions at the United Nations Security Council, set to meet later Wednesday to condemn the North over its Monday test which shocked the world.

It did not elaborate on the physical measures, but the North's number two leader separately threatened "physical steps" if US pressure continues.

"The issue of future nuclear tests is linked to US policy toward our country," Kim Yong-Nam, the head of the North's Supreme People's Assembly, told Japan's Kyodo news agency.

"If the United States continues to take a hostile attitude and apply pressure on us in various forms, we will have no choice but to take physical steps to deal with that."

The South's President Roh Moo-Hyun has promised a stern but calm response to the test, but has not announced what countermeasures he plans. South Korea and the United States say they do not plan a military response.

"We hope the countries concerned through close consultations will deliver a clear and firm message to North Korea in a unified voice," Vice Foreign Minister Lee Kyu-Hyung said of the UN debate.

Prime Minister Han Myeong-Sook told parliament South Korea will observe likely UN Security Council economic sanctions.

While Seoul's diplomatic language has been measured, anger and tension have risen over what many see as North Korea's betrayal.

An apparent offshore earthquake sparked Japanese media reports about a second nuclear test Wednesday morning, which were later dismissed.

A newspaper poll found 78 percent of respondents believed South Korea should revise its policy of engagement, and 65 percent said it should develop nuclear weapons, according to the JoongAng newspaper.

Groups of protesters have held nightly candlelight vigils.

"We worry about a war because the regime is so thoughtless," said one demonstrator, librarian Lee Young-Sook. "The South Korean people are being held hostage."

Roh has admitted the engagement approach is under threat. The South has spent 1.3 trillion won (1.37 billion dollars) since 2003 on its "sunshine" policy.

The main opposition Grand National Party has demanded industrial and tourism projects with the North be shut down.

The architect of the policy, former president Kim Dae-Jung, defended the reconciliation approach and accused the US administration of failures that led to Pyongyang's nuclear test.

"The nuclear test by the North proved that the US policy toward the North had been ill-conceived," Kim said. "Now, the United States must not aim at regime change there but engage in a give-and-take deal with the North."

Aid and rights groups warned of the threat of dwindling support for appeals to feed North Korea and said the coming winter could worsen hunger and refugee flows.

"North Korea's nuclear weapons programme can have devastating security implications in the region," said Sophie Richardson of Human Rights Watch. "But suspending food aid could be lethal for ordinary North Koreans."

Meanwhile, South Korean civic groups shipped aid of 2,000 bicycles, two ambulances, blankets and boilers to North Korea late Wednesday while the Seoul government still suspended its shipment of humanitarian aid.

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