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Kim Jae-hwan, Getty Images
South Korean protesters burn a North Korean flag and portraits of North Korean leader Kim Jong-Il, condemning a nuclear test.

Facing the first direct challenge to his new administration by an emerging nuclear-weapons state, President Barack Obama declared Monday that the United States and its allies would "stand up" to North Korea, hours after that country defied international sanctions and conducted what strongly appeared to be its second nuclear test.

Obama reacted to the underground blast as White House officials scrambled to coordinate an international response to a North Korean nuclear capability that none of his predecessors had proved able to reverse.

The United Nations swiftly condemned North Korea for its test, and South Korea announced today that it will join a U.S.-led initiative to intercept ships suspected of spreading weapons of mass destruction.

The U.N. Security Council said the test was a "clear violation" of a 2006 resolution banning North Korea from conducting nuclear development, and that it would start work immediately on a new resolution that could result in even stronger measures.

Russian officials said the nuclear bomb that the North detonated underground Monday was comparable to those that obliterated Hiroshima and Nagasaki, raising fears that the communist country could spread such technology abroad.

In a further sign of the North's mounting standoff with the world, a report said the country was likely preparing to fire short-range missiles today off its western coast.

Obama told South Korean President Lee Myung-bak that the United States will protect South Korea from any possible North Korean aggression and called for a "strong resolution" by the U.N., Lee's spokesman Lee Dong-kwan said after the two leaders spoke by telephone today.

South Korea, which previously stayed out of the U.S.-led Proliferation Security Initiative in order to pursue reconciliation efforts with North Korea, set aside its reservations and announced it would join the pact immediately. The program involves stopping and searching ships suspected of carrying nuclear, chemical and biological weapons, materials to make them, or missiles to deliver them.

North Korea previously has warned the South that its joining the program would be considered an act of war.

Acutely aware that their response to the explosion in the mountains of Kilju, not far from the Chinese border, would be seen as an early test of a new administration, Obama and his aides said they were determined to organize a significantly stronger response than the Bush administration had managed after the North Korea's first nuclear test, in October 2006.

Speaking in the Rose Garden after returning to the White House from Camp David and meeting his top aides in the Oval Office, Obama vowed to "take action" in response to what he called "a blatant violation of international law" and the North Korea's declaration that it was repudiating past commitments to dismantle its nuclear program.

But as they had meetings every few hours — including a lengthy session in the Situation Room on Monday evening — some of Obama's aides acknowledged that the administration's options were limited.

Much depends, they said, on the new president's ability to convince Russia and China to go significantly beyond the strong condemnations that they issued Monday against North Korea, their former ally as a vestige of Cold War Communism.

"I think we were all impressed with the fact that the Russians and the Chinese denounced this so strongly," Rahm Emanuel, Obama's chief of staff, said in a telephone call.

Yet turning that into effective action will prove a challenge.

Obama must now decide how to mix what he called "stronger international pressure" with a new set of diplomatic overtures, at a moment when, his aides are acutely aware, Iran and other nations are taking his measure, examining the confrontation with North Korea for hints of how he will handle complex confrontations to come.

It will take days, or weeks, of testing radioactive particles vented into the atmosphere to calculate the size of the device detonated Monday, and even then, there will be continuing debate about whether North Korea has the engineering capability to make a weapon compact enough to fit in the warhead of a missile, much less to deliver it to a target.

Japanese and South Korean officials acknowledged they are less concerned about direct attack from North Korea — which would almost certainly result in a devastating, U.S.-led response — than North Korea playing its last card: Selling its twice-tested nuclear-weapons technology on the black market, much as it has sold its missile and reactor technology in the Middle East.

"We're back to the same problem Bush had," said one exasperated intelligence official. "The threat is not that they will shoot off a nuclear weapon, it's that they will sell nuclear material."

In emergency conference calls just after the North gave less than an hour's notice through its mission to the United Nations that it was about to conduct a test early Monday, Obama's team came to some preliminary strategies.

One senior administration official said that the United States would never grant full diplomatic recognition to North Korea — or sign a peace treaty formally ending the Korean war — unless its nuclear capability is dismantled.

To devise a common response, administration officials began planning a series of trips to meet with Asian leaders, and eventually with the central player in the diplomatic drama: China. Defense Secretary Robert M. Gates will begin the effort this week on a previously scheduled trip for an annual defense meeting. His spokesman, Geoff Morrell, said, "There is simply no greater security challenge facing Asia than a nuclear-armed North Korea," and Gates plans to work "to figure out how we collectively can prevent that from becoming a reality."

Contributing: William J. Broad, Thom Shanker, Mark Landler and Neil MacFarquhar, New York Times News Service; Hyung-Jin Kim, Associated Press