N. Korea claims nuclear test success

North Korea is boasting that it performed its first-ever nuclear weapons test today, saying it detonated a successful underground blast in a " great leap forward" - in defiance of international warnings.

The reported nuclear test sparked worldwide condemnation against the communist regime and concerns it could seriously destabilize the region, with even Pyongyang's ally China strongly opposing the move. The US called for immediate UN Security Council action, and along with Japan was expected to press for more sanctions on the impoverished North.

The North's official Korean Central News Agency said the underground test was performed successfully "with indigenous wisdom and technology 100 percent," and that no radiation leaked from that test site.

"It marks a historic event as it greatly encouraged and pleased the (Korean People's Army) and people that have wished to have powerful self-reliant defense capability," KCNA said, adding that it was "a great leap forward in the building of a great prosperous powerful socialist nation."

If the test is confirmed, North Korea would be the ninth country known to have nuclear weapons, along with the United States, Russia, France, China, Britain, India, Pakistan and Israel.

A nuclear North Korea would dramatically alter the strategic balance of power in the Pacific region and seriously undermine global anti-proliferation efforts.

Australia and South Korea said there was seismic confirmation that pointed to a nuclear test, and a top Russian military officer confirmed the device tested was a nuclear weapon, the ITAR-Tass news agency reported. However, Japan and the United States said they couldn't immediately confirm a test.

South Korea's seismic monitoring center said a magnitude 3.6 tremor felt at the time of alleged North Korea nuclear test wasn't a natural occurrence.

The size of the tremor could indicate an explosive equivalent to 550 tons of TNT, said Park Chang-soo, spokesman at the Korea Institute of Geoscience and Mineral Resources — which would be far smaller than the nuclear bombs the US dropped on Japan in World War II.

The atomic bomb that struck Hiroshima, Japan on Aug. 6, 1945, had the destructive power of about 15,000 tons (33 million pounds) of TNT.

The US Geological Survey said it recorded a seismic event with a preliminary magnitude of 4.2 in northeastern North Korea coinciding with the test claim, but survey official Bruce Presgrave said the agency was unable to tell if it was an atomic explosion or a natural earthquake.

The White House said a test defied world opinion against it.

"A North Korean nuclear test would constitute a provocative act in defiance of the will of the international community and of our call to refrain from actions that would aggravate tensions in Northeast Asia," White House spokesman Tony Snow said.

"We expect the UN Security Council to take immediate actions to respond to this unprovoked act," Snow said. "The United States is closely monitoring the situation and reaffirms its commitment to protect and defend our allies in the region."

A Security Council resolution adopted in July after a series of North Korean missile launches imposed limited sanctions on North Korea and demanded the country rejoin international nuclear talks. The North immediately rejected the plea.

Also at the UN, South Korea's Foreign Minister Ban Ki-moon was expected later Monday to be nominated as the next secretary-general of the United Nations by the Security Council. Ban has said he would use the post, which he would assume at year's end, to press for a resolution of the North Korean nuclear standoff.

Britain's Prime Minister Tony Blair said the test was a "completely irresponsible act," and its Foreign Ministry warned of international repercussions.

Japanese Prime Minister Shinzo Abe, in Seoul for a summit meeting, said the test had yet to be confirmed but that it would call for a "calm yet stern response."

On Sunday in Beijing, Abe and Chinese President Hu Jintao had pledged to work together to avert a North Korean test.

China, the North's closest ally, said on Monday that Beijing "resolutely opposes" the North Korean nuclear test and hopes Pyongyang will return to disarmament talks.

South Korean President Roh Moo-hyun said the test would make it difficult for Seoul to maintain its engagement policy with its communist neighbor.

"This is a warning as well as my prediction," Roh told journalists after his summit with Abe. "Under this situation, it's difficult for South Korea to maintain engagement policy."

The two Koreas, which fought a 1950-53 war that ended in a cease-fire but no peace treaty, are divided by the world's most heavily armed border. However, they have made strides toward reconciliation since their leaders met at their first-and-only summit in 2000.

The South had planned to ship 4,000 tons of cement to the North on Tuesday as emergency relief following massive flooding there in mid-July, but decided to delay it after the test, Yonhap news agency reported, quoting an unidentified Unification Ministry official.

"South Korea won't be patient for everything, make concessions on everything and accept all demands from North Korea as it did in the past," Roh said.

Impoverished and isolated North Korea has relied on foreign aid to feed its 23 million people since its state-run farming system collapsed in the 1990s following decades of mismanagement and the loss of Soviet subsidies.

South Korea's Defense Ministry said the alert level of the military had been raised in response to the claimed nuclear test, but that it noticed no unusual activity among North Korea's troops.

The repercussions of North Korea's announcement also were felt in financial markets, sparking plunges in South Korea's stocks and its currency, the won.

The test could further harden international consensus against North Korea, said Toshimitsu Shigemura, a North Korea expert at Tokyo's Waseda University.

"Japan and the United States will further strengthen overall cooperation ... including missile defense," he said. "The nuclear test will also bring Japan, China and South Korea closer, though political differences will remain."

The North has refused for a year to attend international talks aimed at persuading it to abandon its nuclear ambitions. The country pulled out of the Nuclear Nonproliferation Treaty in 2003 after US officials accused it of a secret nuclear program, allegedly violating an earlier nuclear pact between Washington and Pyongyang.

The North is believed to have enough radioactive material for about a half-dozen bombs. It insists its nuclear program is necessary to deter a US invasion.

The North also has active missile programs, but it isn't believed to have an atomic bomb design small and light enough to be mounted on a long-range rocket that could strike targets as far as the US

In Pyongyang, North Koreans went about their lives as usual Monday with no signs of heightened alert by security forces. Red flags of the North's Korean Workers' Party draped buildings and lampposts to mark Tuesday's 61st anniversary of the party's founding.

The country's state TV read the official report about the test during a regular newscast. The item wasn't the top story, and there were no images shown of the test.