UNITED NATIONS – The world lined up against North Korea on Monday for staging a nuclear test denounced even by key allies. President Bush called it “a threat to international peace and security,” and the U.N. Security Council began weighing severe sanctions to punish the impoverished communist nation.

There was no talk of military action. But the Security Council quickly condemned North Korea’s decision to flout a U.N. appeal to cancel the test after the reclusive regime announced it had set off an underground atomic explosion.

Russia was the only country to say it had “no doubts” over the North Korean claim, but the U.S. and other experts said the explosion was smaller than expected and they had yet to confirm it was nuclear.

But the reaction of world governments reflected little doubt that they were treating the announcement as fact.

The 15-nation council urged Pyongyang to return to stalled talks, refrain from further tests and keep its pledge to scrap its clandestine weapons program.

Bush said the North Korean action “constitutes a threat to international peace and security” and requires “an immediate response” from the Security Council, though he stressed the U.S. remained committed to diplomacy.

U.S. Ambassador John Bolton and key allies Britain and France also said they would seek tough new U.N. sanctions against the North. The U.S. proposal is aimed at curbing its nuclear and ballistic missile programs and would prohibit all trade in military and luxury goods and prevent “abuse of the international financial system.”

Japan’s U.N. Ambassador Kenzo Oshima, the current council president, said all council members “emphasized that the response of the council should be strong, swift and very, very clear in its message and its action.”

But just how long it will take members to agree on a resolution remains to be seen.

Council experts started discussing the proposals in meetings Monday afternoon, but it was unclear whether China and Russia would support some of the tough measures. They include international inspection of all cargo to and from North Korea to limit the proliferation of nuclear, chemical and biological weapons, blanket bans on luxury and military goods, and any material that could be used in the production of weapons of mass destruction.

South Korea said the nuclear test was conducted at 9:36 p.m. EDT Sunday in Hwaderi on the northeast coast.

Before the experts meeting, the ambassadors from the five veto-wielding council nations met with Oshima.

Bolton told reporters afterward that everybody agreed within 30 minutes that the council should condemn the action and respond quickly, saying “that’s remarkable” to have such a unanimous decision.

But he wouldn’t speculate when the council might act, noting that Japan and others already had other suggestions for the text.

“The fact is that in our half-hour, full council meeting this morning, there was no one who even came close to defending this test by North Korea,” Bolton said.

The United States, France, Britain and Japan want the resolution under Chapter 7 of the U.N. Charter, which deals with threats to international peace, breaches of the peace and acts of aggression. It allows the council to authorize measures ranging from breaking diplomatic ties and imposing economic and military sanctions to taking military action to restore peace.

With U.S. forces strapped by the twin wars in Iraq and Afghanistan, the Bush administration repeatedly has said it has no plans to invade North Korea and discussion of military action was absent on Monday.

Neither Russia nor China would say whether they support a resolution that could pave the way for sanctions.

“I think we have to react firmly,” China’s U.N. Ambassador Wang Guangya said. “But also I believe that on the other hand the door to solve this issue from a diplomatic point of view is still open.”

Russia’s U.N. Ambassador Vitaly Churkin said the North Koreans “will be facing a very serious attitude on the part of the Security Council and the entire international community,” but he said the council needs to discuss whether that will include sanctions.

The reported test came one day after the ninth anniversary of reclusive North Korean leader Kim Jong Il’s accession to power and a day before the 61st anniversary of the ruling North Korean Workers’ Party.

AP Television News footage showed North Koreans going about their daily business and there were no signs of heightened alert by security forces in Pyongyang, hours after their government said it performed a nuclear weapons test.

People also laid flowers by a statue of Kim Il Sung, the current leader’s father who died in 1994. Red flags of the party draped buildings and lampposts.

The test also coincided with the Security Council vote Monday to nominate South Korean Foreign Minister Ban Ki-moon to succeed Kofi Annan as the next U.N. secretary-general. The 192-nation General Assembly is expected to approve the recommendation later this month.

Ban said one of his priorities, if approved, would be to work to resolve the North Korean crisis.

North Korea remained defiant. Pak Gil Yon, the North’s U.N. ambassador, said the Security Council should congratulate the Democratic People’s Republic of Korea, known as the DPRK, instead of passing “useless” resolutions or statements.

“The nuclear test in the DPRK will greatly contribute in increasing the world deterrence of the DPRK” and will contribute “to the maintenance and guarantee of peace and security in the peninsula and the region,” he said.

The United States and its allies, and many of North Korea’s neighbors, took the exact opposite view.

“This shows why we need actions and not just words about North Korea,” Bolton told The Associated Press.

Although North Korea has long claimed it had the capability to produce a bomb, the test would be the first proof that it had done so.

If the test is confirmed, North Korea would join the current members of the nuclear club _ the United States, Russia, Britain, France, India, Pakistan and China. Israel is widely believed to have the bomb but has not publicly declared.

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

“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,” Japanese Prime Minister Shinzo Abe said at a news conference in Seoul after a summit with South Korean President Roh Moo-hyun.

Abe, facing his first major foreign policy test since his recent election, called for a “calm yet stern response.” Japanese Foreign Minister Taro Aso warned such a test would “severely endanger not only Northeast Asia but also the world stability.”

South Korea said it had put its military on high alert, but it had noticed no unusual activity among North Korea’s troops.

Bolton told the Security Council that Washington would consider an attack on Japan or South Korea an attack on the United States, according to U.N. diplomats who spoke on condition of anonymity because the remarks were made at a closed council meeting.

The United States has defense agreements with the two Asian allies and thousands of U.S. troops are stationed in South Korea and Japan.

China said the North “defied the universal opposition of international society and flagrantly conducted the nuclear test” and urged the North to return to six-party nuclear disarmament talks.

Russian President Vladimir Putin told his Cabinet that Moscow “certainly condemns the test conducted by North Korea.”

British Prime Minister Tony Blair said the test was a “completely irresponsible act.”

Only Iran, which also faces Security Council action over its disputed nuclear program, expressed understanding for North Korea’s action.

Iranian state radio blamed the North’s reported nuclear test on U.S. pressure, saying the test “was a reaction to America’s threats and humiliation.”

The North has refused for over a year to attend six-party international talks aimed at persuading it to disarm, demanding instead that the U.S. drop financial sanctions it has imposed to punish Pyongyang alleged counterfeiting and money laundering. It 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.

Impoverished and isolated, North Korea has built up its military and nuclear programs while relying 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 said the nuclear test was conducted at 10:36 a.m. Monday (9:36 p.m. EDT Sunday) in Hwaderi near Kilju city on the northeast coast.

The North’s official Korean Central News Agency said the test was successful, with no leak of radiation, and this was “a stirring time when all the people of the country are making a great leap forward in the building of a great prosperous powerful socialist nation.”

“It marks a historic event as it greatly encouraged and pleased the … people that have wished to have powerful self-reliant defense capability,” KCNA said. “It will contribute to defending the peace and stability on the Korean Peninsula and in the area around it.”

The North is believed to have enough radioactive material for 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 distant as the United States.

Reports about the size of the explosion were conflicting, ranging from South Korea’s geological institute estimating it was the equivalent of 550 tons of TNT to Russia’s Defense Minister Sergei Ivanov saying it was the equivalent of 5,000 to 15,000 tons of TNT.

A U.S. government official, who spoke on condition of anonymity because of the political sensitivity of the situation, said the seismic event could have been a nuclear explosion, but its small size was making it difficult for authorities to pin down.

The U.S. Geological Survey said it recorded a magnitude 4.2 seismic event in the northeastern part of the country. Asian neighbors also said they registered a seismic event, and an official of South Korea’s monitoring center said the magnitude 3.6 tremor wasn’t a natural occurrence.

Japan dispatched three aircraft to waters between Japan and the Korean peninsula to monitor radiation levels, the Defense Agency said. Russia reported no increase in radiation levels in its Primorye territory, which borders North Korea.