North Korea's detonation of a nuclear device Monday appears not to have been a significant technical advance over its first underground test three years ago. But it has triggered a swifter, stronger and more uniform wave of international condemnation, most notably from the isolated nation's historical allies, China and Russia.
In the United States, President Barack Obama, facing the first nuclear weapons challenge to his new administration by an emerging nuclear weapons state, accused North Korea of "blatant violation of international law." White House officials scrambled to coordinate an international response to a North Korean nuclear capability that none of his predecessors were able to reverse.
The U.N. Security Council moved quickly in an emergency meeting to condemn the test, saying it constituted a clear violation of a 2006 U.N. resolution barring the Communist state from exploding a nuclear weapon. The council's speedy response contrasted with protracted discussions after North Korea's April 5 launch of a long-range missile.
China's response Monday was significantly more pointed than previous reactions, saying it was "resolutely opposed" and telling Pyongyang to avoid actions that heighten tensions. The explosion in the mountains of Kilju was not far from the Chinese border.
The test escalates a pattern of provocation that this spring has included launching the long-range missile, detaining two U.S. journalists, kicking out U.N. nuclear inspectors, restarting a plutonium factory and halting six-nation nuclear negotiations.
North Korea said its second nuclear test was more powerful and better controlled than its 2006 test, which many experts characterized as a semifailure. But several U.S. experts on nuclear weapons said Monday's test demonstrated that the North Koreans have not yet mastered the technology of creating a reliable nuclear bomb, though they expressed little doubt that the regime had exploded a nuclear device.
"The simplest hypothesis is that they're trying to build a weaponizable device and they're still not that good at it," said Jeffrey Lewis, director of the Nuclear Strategy and Nonproliferation Initiative at the New America Foundation, a nonprofit group.
North Korea has for years been the target of international sanctions intended to limit the country's access to bomb and missile-making technology. But a senior administration official said that although the sanctions have undermined the North's economy, they have had little direct effect on its "entirely indigenous" nuclear program. The government mines its own uranium, builds laboratories using its own technical expertise and generates its own plutonium.
After it exploded the small nuclear device in 2006, North Korea agreed to begin shutting down its main nuclear reactor and began to disable it. It did so in return for food, fuel and diplomatic concessions. But the negotiations did nothing to stop North Korea from trying to improve the quality of its nuclear devices.
Analysts said the test may also be related to succession issues inside North Korea. Last summer Kim Jong Il reportedly suffered a stroke. "He may be impatient," said Koh Yu-whan, a professor of North Korean studies in Seoul.
Japanese Prime Minister Taro Aso said his country "absolutely cannot tolerate" the nuclear test because North Korea is also beefing up its ballistic missile capability, which "could be a means of transportation for weapons of mass destruction."
Japan's anxiety about the test is heightened by its vulnerability to attack from nearby North Korea, which has more than 200 midrange Nodong missiles capable of striking most of the country.
South Korea, which had stayed out of the U.S.-led Proliferation Security Initiative to pursue reconciliation with North Korea, said it would immediately join the program, which involves intercepting ships suspected of spreading weapons of mass destruction. North Korea had warned the South that joining would be considered an act of war.
South Korea's Yonhap news agency said North Korea banned ships off its western coast and would probably fire short-range missiles as early as today.