World leaders condemned North Korea for carrying out nuclear and missile tests and U.S. President Barack Obama said Pyongyang's actions were a reckless challenge warranting action from the international community.

As nations prepared for emergency U.N. Security Council talks on Monday, European Union foreign policy chief Javier Solana branded the test a flagrant violation of a Council resolution which required a firm response.

China, Russia, France and Britain -- which with the United States are the permanent Council members -- expressed alarm at the hermit state's test that Moscow said was as powerful as the U.S. atom bomb dropped on Nagasaki in World War Two.

South Korea called an emergency cabinet meeting.

By acting in blatant defiance of the United Nations Security Council, North Korea is directly and recklessly challenging the international community, Obama said. The danger posed by North Korea's threatening activities warrants action by the international community.

The uniform condemnation from around the globe underscored the isolation of the secretive state.

Nations for years have tried a mix of huge aid pledges and tough economic sanctions to persuade impoverished North Korea to give up its nuclear weapons program.

Pyongyang, however, is so isolated there are few punishment options open and neither Obama nor Solana made specific recommendations. France spoke of strengthening sanctions.

North Korea's behavior increases tensions and undermines stability in Northeast Asia. Such provocations will only serve to deepen North Korea's isolation, Obama said. It will not find international acceptance unless it abandons its pursuit of weapons of mass destruction and their means of delivery.

U.N. Secretary-General Ban Ki-moon said he was deeply worried by the development and Japan said it would seek a new resolution condemning the test at the Security Council meeting.

North Korea had for weeks threatened to conduct the test in response to tighter international sanctions for its April launch of a rocket. Pyongyang also said it would no longer be a party to six-country talks on giving up its nuclear weapons program.

ERRONEOUS, MISGUIDED

Russia said the violation of the Security Council resolution was a serious blow to efforts to control the spread of nuclear weapons and that six-party talks on North Korea were the only solution to the crisis.

The latest steps by North Korea escalate tensions in Northeast Asia and endanger security and stability in the region, Russia's foreign ministry added in a statement.

Monday's blast was up to 20 times more powerful than the North's first nuclear test, underscoring the advances in its nuclear program. Ratcheting up tensions further, North Korea test-fired three short-range missiles just hours later.

We consider it a provocation and we strongly condemn them, European Commission President Jose Manuel Barroso said.

British Prime Minister Gordon Brown described the nuclear and missile tests as erroneous, misguided and a danger to the world, saying the international community would treat North Korea as a partner only if it behaved responsibly.

France said it would be consulting its Security Council partners on the consequences to draw from this serious act by North Korea, and in particular on strengthening sanctions.

The test was greeted with concern in the region.

On the basis that North Korea has conducted a nuclear underground explosion, they deserve and get nothing other than our absolute condemnation, and that condemnation should be echoed around our region and the globe, Australian Foreign Minister Stephen Smith told parliament.

China, which as a permanent Council member could veto any resolution, called on Pyongyang not to make the situation worse. But political analysts said Beijing was unlikely to back stronger sanctions.

The Chinese side vehemently demands North Korea abides by its denuclearization promises, stop any actions which may worsen the situation and return to the six-party talks process, China said in a statement. The Chinese government calls on all sides to calmly and appropriately deal (with the situation).

China has long seen the North as a strategic buffer against the extension of U.S. forces up to its border. Beijing's policy makers are juggling concerns about potential instability in North Korea, its weakened sway there and fears of a regional confrontation over Pyongyang's nuclear arms program.

(Writing by Peter Millership; Editing by Richard Balmforth)

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