The United States on Wednesday stressed its resolve to defend allies Japan and South Korea amid what it called North Korean "saber-rattling and bluster" in the face of growing global isolation.

One day after drawing international condemnation of its latest nuclear and missile tests, the regime of Kim Jong-Il said it would abandon the truce that ended the Korean war and warned it could launch a military attack on the South.

"I want to underscore the commitment the United States has, and intends always to honor, for the defense of South Korea and Japan," US Secretary of State Hillary Clinton declared in response.

At the White House, spokesman Robert Gibbs warned that "threats won't get North Korea the attention it craves. Their actions are continuing to further deepen their own isolation from the international community."

"We're certainly concerned and take any threat seriously. But my sense is they're trying to get renewed attention through saber-rattling and bluster and threats," Gibbs told reporters.

Clinton said the hermetic Stalinist regime was in violation of its pledges under six-party denuclearization talks with China, Japan, Russia, South Korea and the United States and warned of "consequences to such actions."

The top US diplomat said "a unified international community, including China and Russia," was crafting a United Nations response to punish North Korea, "rein in" the regime, and resume efforts to dismantle its nuclear arsenal.

At the Pentagon, a US defense official said there were no signs that North Korea had restarted work at its nuclear fuel reprocessing plant north of Pyongyang to make more weapons-grade plutonium.

The official, who requested anonymity, told AFP he was not aware of any resumed activity at the plant in the Yongbyon complex.

The comment came after South Korean media reports, citing US spy satellite images, that steam had been seen coming from the facility, suggesting it had been reactivated.

But commercial satellite photos released Wednesday showed no steam coming from the plant, a US arms control institute said.

North Korea, which uses a coal-fired plant to generate steam for work at the reprocessing plant, had announced April 14 it was quitting a July 2007 nuclear deal under the six-party talks and would reopen the Yongbyon plant.

The scrutiny came amid escalating rhetoric from North Korea, which said Wednesday 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 North's anger was provoked by the South's decision to join a US-led international security initiative, established after the September 11, 2001 terrorist attacks to stop the spread of weapons of mass destruction.

The newly bellicose rhetoric drew a cautious reaction at the White House, where some saw North Korea's actions as a test of President Barack Obama and international cohesion on efforts to dismantle the North's nuclear weapons program.

"This is the fifth time in 15 years that they've sought to nullify the armistice governing the Korean war," said Gibbs, adding: "I think their actions would be better focused on living up to their rights and obligations."

The United States and its allies are doing "all that we can" to ensure North Korea is not spreading nuclear know-how, said the spokesman.

But amid the bluster, Clinton offered a diplomatic olive branch to Pyongyang, leaving open its return to the negotiating table.

"There will be an opportunity for North Korea to come back into a framework of discussion within the six-party process and that we can begin once again to see results from working with the North Koreans toward denuclearization."

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