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SEOUL, South Korea (CNN) -- North Korea delivered on its threat Monday, conducting a second nuclear test that angered governments around the globe.

The North had threatened to do so unless the U.N. Security Council apologized for imposing sanctions on it following a rocket test on April 5.

The secretive communist state also apparently test-fired a short-range missile on Monday, the White House said.

Japan called for an emergency meeting of the U.N. Security Council. One is expected Monday, said Vitaly Churkin, the Russian ambassador to the United Nations.

The White House -- which less than three weeks ago announced a new diplomatic effort to restart stalled talks with North Korea about its nuclear program -- said the test was in "blatant defiance" of the Security Council.

"North Korea is directly and recklessly challenging the international community," the White House said. "The danger posed by North Korea's threatening activities warrants action by the international community."

European Union foreign policy chief Javier Solana condemned the test.

"These irresponsible acts by North Korea warrant a firm response by the international community. The European Union will be in contact with its partners to discuss appropriate measures," he said in a written statement.

UK Prime Minister Gordon Brown said: "I condemn North Korea's nuclear test in the strongest terms. This act will undermine prospects for peace on the Korean peninsula and will do nothing for North Korea's security. The international community will treat North Korea as a partner if it behaves responsibly. If it does not then it can expect only renewed isolation."

North Korea announced its underground nuclear test a little more than an hour after the U.S. Geological Survey reported a magnitude 4.7 seismic disturbance at the site of North Korea's first nuclear test.

There was no immediate information on the yield of the weapon used in Monday's test. The Russian Defense Ministry said the explosion was between 10 to 20 kilotons. The U.S. State Department said it was analyzing the data.

The North's state-run Korean Central News Agency would say only that the latest test was safely conducted "on a new higher level in terms of its explosive power and technology of its control."

U.S. intelligence estimated that the first North Korean test -- in October 2006 -- produced an explosion equal to less than 1,000 tons of TNT. The low yield was a fraction of the size of the bombs the United States dropped on Japan at the end of World War II.

Intelligence analysts had predicted North Korea would conduct a second rocket or nuclear test.

The North threatened to do so after the U.N. Security Council voted unanimously to condemn its April launch as a violation of a 2006 resolution that bans North Korea's ballistic missile activity.

North Korea insisted the rocket was a communications satellite. It retaliated by threatening to walk away from the six-party talks aimed at disarming the country of nuclear weapons.

The talks -- involving China, Japan, North Korea, Russia, South Korea and the United States -- have been aimed at persuading North Korea to scrap its nuclear program.

The North also expelled international inspectors from its nuclear sites, announced plans to restart the reactor that produces plutonium for its nuclear weapons, and threatened to launch more rockets and another nuclear device.

Monday's test was conducted "as part of the measures to bolster up its nuclear deterrent for self-defense in every way," the North Korean news agency said.

"They think this will enhance and embolden their diplomatic capability to deal with the other countries," said Han Sung Joo, former South Korean ambassador to the United States. "Of course, it may be working the other way. But from their point of view, this is their lifeline, which they want to maintain."

North Korea's latest move could clear the way for the United States and the other members of the six-party talks, minus North Korea, to impose new punitive measures against a country desperate for food and energy assistance.

For now, however, the North's nuclear arms program is not a major security threat, analysts say. The country has yet to build an effective bomb or develop an effective delivery system to a target country.

Last year, North Korea acknowledged producing roughly 88 pounds (40 kilograms) of enriched plutonium -- enough for about seven nuclear bombs.

Analysts say North Korea is years from having a weapon it can put atop a long-range missile like those in the U.S., Chinese or Russian arsenals.

"I know a lot of people may think, 'Oh no, a nuclear test. Does that mean war, conflict in the Korean Peninsula?'" said Jim Walsh, an international security analyst at the Massachusetts Institute of Technology.

"The answer is 'no.'"

There was no immediate information on the yield of the weapon used in Monday's test. The Russian Defense Ministry said the explosion was between 10 to 20 kilotons. The U.S. State Department said it was analyzing the data.