SEOUL - North Korea on Wednesday threatened a new nuclear test unless the U.N. Security Council apologized for tightening sanctions, confirming some analysts' fears that Pyongyang is determined to build an atomic arsenal.

The United States said such threats were counterproductive and urged Pyongyang to return to stalled six-nation denuclearization talks.

I don't think you will see an apology from the Security Council. Let me just say very clearly that these threats only further isolate the North, U.S. State Department spokesman Robert Wood said.

The North needs to stop making these threats, he told reporters in Washington.

The threat adds to tension in East Asia after North Korea's April 5 rocket launch that the United States and other nations said was the disguised test of a long-range missile.

In case the (U.N. Security Council) does not make an immediate apology ... the DPRK (North Korea) will be compelled to take additional self-defensive measures in order to defend its supreme interests, the North's foreign ministry spokesman was quoted as saying by the KCNA news agency.

The measures will include nuclear tests and test-firings of intercontinental ballistic missiles.

A Japanese official called for a calm response. Japanese Prime Minister Taro Aso, speaking without mentioning the latest threat, said the protracted six-party talks on North Korea, which the Pyongyang has abandoned, were the way forward.

North Korea is strongly opposed to the international community's message, and we are taking into consideration the possibility of the North raising tensions, Aso was quoted by a Japanese government official as telling Chinese Premier Wen Jiabao in Beijing.

But we think that it is important not to overreact and to respond calmly, he was quoted as saying. Japan thinks that the six-party talks are the most realistic framework to push forward the denuclearization of North Korea. The talks join Japan, China, the two Koreas, Russia and the United States.

CHINESE INFLUENCE

North Korea timed its latest threat just before the talks in Beijing between Aso and Wen at which Aso also urged China, host of the six-party talks, to play an important role with its great influence over North Korea.

Wood urged countries with influence over Pyongyang, a reference to China, to convince North Korea to live up to their obligations.

China is the closest North Korea has to a major ally, but a nuclear test by the North would be seen as destabilizing the region and is likely to antagonize Beijing.

A fresh nuclear test would rattle financial markets in Seoul and Tokyo but the impact could be short-lived because much of the risk has already been factored in.

The Security Council imposed sanctions on North Korea following a ballistic missile launch in July 2006 and a nuclear test a few months later. After this month's rocket launch, it called for the sanctions to be tightened.

Impoverished North Korea has lashed out at the measures, saying it would boycott the six-party disarmament talks and bolster its nuclear deterrent.

At the weekend, it said it had started extracting plutonium from spent fuel rods at its nuclear arms plant.

That announcement came hours after a U.N. Security Council panel placed three North Korean firms on a blacklist for aiding Pyongyang's missile and nuclear programs.

The fundamental fact is North Korea is set on becoming a nuclear power and to accomplish that, more tests are needed, said Zhang Liangui, an expert on the North at the Central Party School in Beijing. One nuclear test isn't enough. There'll be more nuclear tests and more missile tests.

North Korea has pushed to be recognized as a nuclear state in an attempt that some analysts says is to boost its negotiating position with the United States, a country Pyongyang believes will never otherwise recognize it as a bargaining partner.

Its first nuclear test in 2006 was seen as a partial success at best.

I don't think it's a matter of negotiations. ... North Korea already has a plan. I think they will do it (try to build a nuclear arsenal) anyway, said Park Syung-he, a director of the Asia Strategy Institute in Seoul.

Some other analysts said the North may be merely turning up the rhetoric and that it would be too costly for it to go through with an actual nuclear test.

It's showing its nervousness and feeling antsy about its threats being considered as all barking and no biting after a series of unsuccessful experiments, and that's why it has made a more heightened threat this time, said Lee Dong-bok from the Center for Strategic and International Studies.

North Korea is believed to have produced about 50 kg (110 lb) of plutonium, which experts say would be enough for six to eight bombs. But the North is unlikely to have mastered the technology to miniaturize a nuclear warhead to mount on a missile.

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