SEOUL -- North Korea said Wednesday it was abandoning the truce that ended the Korean war and warned it could launch a military attack on the South, two days after testing an atomic bomb for the second time.

The announcement came amid reports that the secretive North, which outraged the international community with its bomb test Monday, was restarting work to produce more weapons-grade plutonium.

Defying global condemnation, the regime of Kim Jong-Il said it could no longer guarantee the safety of US and South Korean ships off its west coast and that the Korean peninsula was veering back towards war.

The White House said it viewed Pyongyang's threats as "saber-rattling and bluster" that would only deepen its isolation, with spokesman Robert Gibbs saying that "threats won't get North Korea the attention it craves."

US Secretary of State Hillary Clinton meanwhile stressed US commitments to defend South Korea and Japan, saying in Washington "that is part of our alliance commitment that we take very seriously."

The United States still hoped the North would return to multi-party talks on ending its nuclear program, she added.

The North's latest display of anger was prompted by the South's decision to join a US-led international security initiative, established after the September 11 attacks to stop the spread of weapons of mass destruction.

"Those who have provoked us will face unimaginable merciless punishment," said the military statement on the official Korean Central News Agency, blaming Washington and Seoul for the latest turn of events.

The PSI, which now groups 95 nations, provides for stopping vessels to ensure they are not carrying weapons of mass destruction or the components to make them. The South announced it was joining on Tuesday.

"Any tiny hostile acts against our republic, including the stopping and searching of our peaceful vessels... will face an immediate and strong military strike in response," the North Korean statement said.

It said its military would "no longer be bound" by the 1953 armistice that ended hostilities in the Korean war -- in which the United States fought with the South -- because Washington had drawn its "puppet" Seoul into the PSI.

With no binding ceasefire, it said, "the Korean peninsula will go back to a state of war."

It also said the North "will not guarantee the legal status" of five South Korean islands near the disputed inter-Korean border in the Yellow Sea, which was the scene of bloody naval clashes in 1999 and 2002.

Analysts played down the likelihood of a full-scale conflict between North and South Korea but said clashes near the sea border were possible.

The North has taken a harder line with the international community in recent months -- firing a long-range rocket in April, launching five short-range missiles on Monday and Tuesday and conducting its second nuclear test Monday.

Analysts say Kim Jong-Il, 67, is likely carrying out shows of strength to reassert his control in the impoverished state. He reportedly had a stroke in August, which has renewed questions about who might succeed him.

"Kim is trying to impress the cadres and the elite in general... to convince power holders that his family is the one that should be ruling the country," Peter Beck of American University in Washington told AFP on Tuesday.

"It is not unreasonable to conclude that they are no longer interested in nuclear diplomacy," Beck, a Korea expert, said.

Almost six years of six-nation talks have failed to persuade the North to abandon its nuclear programs in exchange for energy aid, diplomatic benefits and security guarantees.

The international community, including the North's main ally China, strongly condemned its latest nuclear test.

The Russian foreign ministry said it had summoned Pyongyang's ambassador and called for the reclusive state to return to the six-party talks.

Earlier Russian Foreign Minister Sergei Lavrov urged a strong United Nations resolution to condemn Monday's nuclear test.

He insisted however that the stand-off with the reclusive state could only be solved through the multi-party talks, saying that North Korea should not be punished "for the sake of punishment" alone.

Diplomats at the UN Security Council, however, said they would need time to agree on a new resolution.

Meanwhile, South Korean reports said that steam was seen coming from a plant at the North's main nuclear facility at Yongbyon -- a sign it was trying to produce more plutonium.