North Korea threatened Wednesday to conduct a second nuclear test unless the United Nations apologises for condemning its rocket launch, sharply raising the stakes in the dispute over its weapons programmes.

Unless the Security Council apologises immediately, the North vowed to take what it called additional defensive measures.

"The measures will include nuclear tests and test-firings of intercontinental ballistic missiles," its foreign ministry said in a statement on official media.

However, the United States warned North Korea on Wednesday that it was only deepening its international isolation, as a revival of talks for Pyongyang's nuclear disarmament appeared increasingly elusive.

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

Wood said Pyongyang was "not living up to its commitments, and we need to do everything we can to get them back to the table."

North Korea's latest announcement further raised regional tensions following its April 5 launch of a long-range rocket. Pyongyang said it put a peaceful satellite into orbit while other nations saw a disguised missile test.

The Security Council condemned the launch and ordered tougher enforcement of sanctions imposed in 2006 after the North's missile launch and first nuclear test earlier that year.

The communist state reacted angrily, announcing it was quitting a six-nation nuclear disarmament pact and restarting the plants at Yongbyon which produced weapons-grade plutonium.

Hours after the UN's first move to toughen sanctions -- a freeze on the foreign assets of three North Korean firms suspected of aiding the missile programme -- the North said work had begun to reprocess spent fuel rods.

Wednesday's statement blasted the assets freeze as an infringement of its sovereignty.

The North tested an atomic weapon in October 2006. But just over four months later it signed a landmark six-nation deal to disable its atomic plants in return for energy aid and other concessions.

Progress on the pact -- which the North has now abandoned -- was stalled by disagreement over how to verify the North's declared nuclear activities.

The statement also announced plans to build a light-water nuclear reactor and said it would start developing technology to produce fuel for it.

The ministry denounced the Security Council for "illegal provocations" by imposing sanctions on the three firms.

Such measures would not work, it asserted, because North Koreans have for decades lived under "sanctions and blockade" by hostile forces.

Hope of denuclearising the Korean peninsula have now gone forever and the situation "is inching to the brink of war by the hostile forces," it said.

Chinese Premier Wen Jiabao and his Japanese counterpart Taro Aso said the world community should not overreact and agreed that the six-nation talks remained the best forum for achieving denuclearisation, a Japanese official at their meeting in Beijing told journalists.

Analysts said the North was putting pressure on the administration of US President Barack Obama for direct dialogue.

Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul, said Pyongyang was declaring that a nuclear test was certain by demanding the impossible from the UN.

Only the US could stop it by proposing bilateral dialogue, Yang told Yonhap news agency.

"North Korea now thinks Obama's policy is not different from that of Bush," said Cheong Seong-Chang of the Sejong Institute think-tank.

"This is a message that the US should come out quickly for direct and high-level dialogue with North Korea. Otherwise, it is saying it will go ahead with a nuclear test," he told AFP.

The North seems impatient, maybe because of Kim Jong-Il's illness, Cheong said. The country's leader is widely reported to have suffered a stroke last August.

"It is telling the US that it wants a package deal on all pending issues. It is saying missiles, nuclear weapons and the issue of building light-water reactors can be discussed simultaneously," Cheong said.

Kim Yong-Hyun, a North Korea studies professor at Seoul's Dongguk University, agreed that the North seemed angry at the lack of US action.

"It can conduct a nuclear test if it wants. However, I still believe North Korea will try to use this as a bargaining chip," he said.

Under an earlier denuclearisation deal reached in 1994 an international consortium began building proliferation-resistant light water reactors in the North to compensate for the shutdown of its Yongbyon atomic complex.

The reactors were unfinished when the deal collapsed in 2002.

earlier related report
US says nuclear threat deepens NKorea's isolation
The United States told North Korea it is only deepening its isolation with vows to conduct a second nuclear test, as analysts said Pyongyang is playing all its cards to force Washington into negotiations.

The communist state late Wednesday sharply raised the stakes in the dispute over its weapons programmes -- vowing nuclear and missile tests unless the United Nations apologises for condemning its recent rocket launch.

The US State Department ruled out any such UN response.

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

"The North needs to come back to the (negotiating) table."

South Korea's foreign ministry spokesman Moon Tae-Young said Thursday members of six-party nuclear disarmament talks "are discussing how to react to the North's recent behaviour."

On Wednesday his ministry expressed "grave concern" at the North's statement, calling it an "outright challenge" to the international community's unified decision.

South Korean officials and media reports said the US special envoy on North Korea, Stephen Bosworth, is considering a Northeast Asian tour next week to discuss ways to persuade the North to rejoin the talks.

Pyongyang's announcement further raised regional tensions after the April 5 rocket launch. The North said it put a peaceful satellite into orbit while other nations saw a disguised missile test.

Condemning the launch, the UN Security Council ordered tougher enforcement of sanctions imposed in 2006 after the North's missile launch and first nuclear test earlier that year.

That test was seen as only partially successful.

The North reacted angrily to the Council statement, announcing it was quitting a six-nation nuclear disarmament pact and restarting the plants at Yongbyon that produced weapons-grade plutonium.

"The North knows very well it is demanding the impossible (from the UN). It's only building up justifications for nuclear and ballistic missile tests," Professor Yang Moo-Jin of Seoul's University of North Korean Studies told AFP.

Given that supplies of reprocessed fissile material are already available, it may be only months away from a second nuclear test, Yang said.

It may take a few months to assemble another long-range rocket, he said.

"However, the timing for all these will depend on how the US will react," he said. "Depending on what will come of negotiations, it may even return to disarmament talks."

Analysts have said the North wants direct negotiations with the United States rather than the six-nation process, which also involves South Korea, Russia, China and Japan.

Paik Hak-Soon of the Sejong Institute think-tank said that as time goes by, the chances of disarming the North through talks are getting slimmer.

"The big problem is that amid the lack of a diplomatic control mechanism, the North will continue expanding its nuclear arsenal, making denuclearisation harder and pushing the region toward a new arms race," he told AFP.

Paik said the North is demanding a new and higher plane for negotiations with Washington to replace the six-party talks, which have been "mired in technicalities" concerning verification since last year.

"The North is pushing for a new package deal to resolve all the issues at the same time," he said.

Kim Yong-Hyun, a North Korea studies professor at Seoul's Dongguk University, said Wednesday the North could conduct a second nuclear test if it chooses.

"However, I still believe North Korea will try to use this as a bargaining chip," he said.