UNITED NATIONS (AP) _ Members of the U.N. Security Council condemned North Korea's claim of a nuclear test and demanded during an emergency meeting Monday that it return to six-party talks on its weapons program, Japan's U.N. ambassador said.

The Security Council will now discuss ``appropriate measures'' to respond to the test, which was a threat to international peace and security, Ambassador Kenzo Oshima said. President Bush said North Korea has ``defied the will of the international community,'' and that the action ``deserves an immediate response'' by the U.N. Security Council.

There were conflicting reports on the size of the blast in northeast North Korea. South Korea said it was relatively small, while Russia said it had been perhaps as powerful as the nuclear bombs the U.S. dropped on Japan during World War II.

The explosion prompted worldwide concern it could seriously destabilize the region, and even Pyongyang's ally China said it strongly opposed the move. South Korea's spy chief said there were possible indications the North was moving to conduct more tests.

The North's official Korean Central News Agency said the underground test was performed successfully using the country's own technology and that no radiation leaked from the site.

The test ``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 in an English-language dispatch, adding that it was ``a great leap forward in the building of a great prosperous powerful socialist nation.''

If details of the test are 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.

Only Russia said the blast was a nuclear explosion but the reaction of world governments reflected little doubt that they were treating the North's announcement as fact.

``We have no doubt that it was a nuclear explosion,'' Russia's Defense Minister Sergei Ivanov said in televised remarks.

A nuclear North Korea would dramatically alter the strategic balance of power in the Pacific region and seriously undermine global anti-proliferation efforts. Japanese Prime Minister Shinzo Abe said the test would mark the beginning of a ``dangerous nuclear age'' in north Asia.

Australia and South Korea said there was seismic confirmation that pointed to a nuclear test. However, Japan and the United States said they couldn't immediately confirm a nuclear test.

South Korea's seismic monitoring center said a magnitude 3.6 tremor felt at the time of the 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 U.S. dropped on Japan in World War II.

The head of South Korea's spy agency said the blast was equivalent to less than 1 kiloton of TNT, South Korea's Yonhap news agency reported. National Intelligence Service chief Kim Seung-kyu also told lawmakers that there were signs of suspicious movement at another suspected test site, Yonhap said.

French Defense Minister Michele Alliot-Marie also said the explosion measured about half a kiloton, but did not confirm it was caused by a nuclear device.

But Russia's Ivanov said the force of the blast was between five and 15 kilotons.

The atomic bomb that struck Hiroshima, Japan, on Aug. 6, 1945, had the destructive power of about 15 kilotons of TNT.

The U.S. 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.

Nuclear blasts give off clear seismic signatures that differentiate them from other explosions, said Friedrich Steinhaeusler, a professor of physics at Salzburg University. Even if the bomb the North Koreans detonated was small, sensors in South Korea would likely be close enough to categorize the explosion as nuclear, he said.

``I think we have to take them at their word. They're not the type of regime to bluff,'' said Peter Beck, Seoul-based analyst for conflict resolution think tank International Crisis Group.

White House spokesman Tony Snow said the U.S. government had not confirmed the claims of a nuclear test but that it ``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.''

The U.S. also called for immediate U.N. Security Council action.

In July, the Security Council unanimously condemned the North in a unanimous resolution after a series of missile launches, imposing limited sanctions and demanding the country rejoin international nuclear talks. The North immediately rejected the resolution.

Also at the Security Council, South Korea's Foreign Minister Ban Ki-moon was expected later Monday to be nominated as the next secretary-general of the United Nations. 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.

Russian President Vladimir Putin told Cabinet officials Monday that Moscow ``certainly condemns the test conducted by North Korea.'' ``It doesn't just concern North Korea, enormous damage has been done to the process of nonproliferation of weapons of mass destruction in the world.''

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

Japan's Abe, in Seoul for a summit meeting, said the ``the development and possession of nuclear weapons by North Korea will in a major way transform the security environment in North Asia and we will be entering a new, dangerous nuclear age.''

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.

``The breaking of a de-facto global moratorium on nuclear explosive testing that has been in place for nearly a decade and the addition of a new state with nuclear weapon capacity is a clear setback to international commitments to move towards nuclear disarmament,'' the head of the International Atomic Energy Agency, Mohamed ElBaradei, said in a statement.

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

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 is reconsidering plans to ship 4,000 tons of cement of emergency relief to the North for floods it suffered in mid-July, a Unification Ministry official said on customary condition of anonymity. Seoul cut off regular aid after the North's July missile launches.

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.

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 U.S. 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 U.S. invasion.

The North 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 U.S.

In Pyongyang, North Koreans went about their lives as usual Monday with no signs of heightened alert. 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 report about the test during its regular newscasts. The item wasn't the top story and there were no images shown of the test.