The UN Security Council was to meet in New York to try to forge a unified international response after Pyongyang said Tuesday it would test the bomb to deter what it called the US threat of nuclear war.
North Korea gave no date for the test and some analysts saw the announcement as an effort to gain leverage with the United States, but nations said they were taking immediate action and that the threat was serious.
Japan's new Prime Minister Shinzo Abe, set to hold his first summits with the leaders of China and South Korea in the coming days, said the world needed to send a united message to Pyongyang's isolated communist regime.
"It is important that the international community send North Korea a message that it must respond to their concerns," Abe said. "The situation will not be resolved until North Korea recognizes this."
China, the main political ally and chief supplier of food and energy to the impoverished North, appealed for "calm and restraint" from the North, while South Korea warned it would cut off vital economic aid if the test went ahead.
"We are taking measures on the assumption that North Korea is more likely (than not) to conduct a nuclear test," South Korean Defense Minister Yoon Kwang-Ung said. "We are working to come up with countermeasures."
North Korea last drew the ire of the international community in July, when it test-fired seven missiles -- one thought to be capable eventually of reaching US soil.
Those launches led to a Security Council resolution that imposed missile-related sanctions on the North, tightening the restrictions on a regime already under tough US financial sanctions.
US President George W. Bush has lumped North Korea in with Iran and Saddam Hussein's Iraq as an "axis of evil," and the United States was quick to join condemnations of its arch-foe's latest announcement.
Australia, a key US ally, summoned the North Korean ambassador to warn of "severe consequences" ahead.
"This is a defiant, impudent act," Australian Prime Minister John Howard said. "It could be dangerous for the region ... There has to be a maximum international response, in a diplomatic way."
Analysts said there was concern that a North Korean weapons test could undermine stability and spur an arms race in the region, while South Korean officials were quick Wednesday to express fears of a nuclear-armed Japan.
"That may provide an excuse for Japan's nuclear armament, which in turn will cause repercussions from China and Russia and lead to a change in the overall balance of power," Vice Foreign Minister Yu Myung-Hwan told the South Korean parliament.
The North said in February 2005 that it had developed a nuclear weapon and US intelligence services believe the country -- one of the most isolated and secretive in the world -- has developed a few crude atomic weapons.
Following several rounds of intense six-nation talks, Pyongyang appeared to have agreed last year to abandon its nuclear weapons programme in exchange for energy and security guarantees.
But after the United States imposed unilateral sanctions over the North's alleged money-laundering and counterfeiting of US currency, Pyongyang said it would boycott further talks until they were lifted.
In its statement announcing the test, North Korea said the threat of sanctions and war from the United States compelled it to test a nuclear weapon as a deterrent.
"The Korean nation stands at the crossroads of life and death," it said. "A people without a reliable war deterrent are bound to meet a tragic death, and the sovereignty of their country is bound to be wantonly infringed upon."
North Korea has repeatedly accused the United States of wanting to attack the country or at least do away with one of the last remaining pure communist regimes in the world.
Japan's UN ambassador Kenzo Oshima, the Security Council president for October, said the 15-nation body would meet to craft a "firm, appropriate response" to what he called a "very serious matter."
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