[For full coverage of nuclear threat, click on [ID:nSP476645])
* North Korea threatens new nuclear and missile tests
* Analysts say threat may be real
* China-Japan summit likely to discuss issue
By Jack Kim
SEOUL, April 29 (Reuters) - North Korea on Wednesday threatened a new nuclear test unless the U.N. Security Council apologises for tightening sanctions, confirming some analysts' fears that Pyongyang was determined to build an atomic arsenal.
The threat adds to mounting tension in East Asia following the North's April 5 rocket launch that the United States and other governments 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."
Japan called for calm response to the threat. "North Korea is strongly opposed to the message from the six-party (nuclear disarmament) talks, but the involved countries need to respond calmly," a Japanese government official told Reuters in Beijing.
North Korea's timed its latest threat just before talks in Beijing between Japanese Prime Minister Taro Aso and Chinese Prime Minister Wen Jiabao.
China is the closest North Korea has to a major ally, but a nuclear test by the North would be seen as destabilising the region and is likely to antagonise Beijing.
The South Korean and Chinese foreign ministries had no immediate comment.
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.
IMPOSED SANCTIONS
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.
The impoverished North has lashed out at the measures, saying it would boycott six-country 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 programmes.
"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 recognised as a nuclear state in an attempt some analysts says is to boost its negotiating position with the United States, a country that Pyongyang believes will never otherwise recognise it as a proper 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. [For a snap analysis on the North's threat, click [ID:nSEO194605]
"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, a senior associate at 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 miniaturise a nuclear warhead to mount on a missile.
North Korea also said it wants to build a light-water nuclear reactor, a long-sought project for the energy-short state. But analysts said the North does not have the technology for the expensive and modern reactor.
(Additional reporting by Junghyun Kim, Jonathan Thatcher and Jon Herskovitz in Seoul, Yoko Kubota, Chris Buckley and Ben Blanchard in Beijing; Editing by David Fox)