WASHINGTON -- North Korea confirmed on Tuesday that it had conducted its third, long-threatened nuclear test, provoking international rebukes, eliciting pledges of further punitive action from the United Nations Security Council and posing a new challenge for the Obama administration in its effort to keep the country from becoming a full-fledged nuclear power.
The official K.C.N.A. news service of North Korea said the country had used a "miniaturized and lighter nuclear device with greater explosive force than previously" and that the test "did not pose any negative impact on the surrounding ecological environment."
Early Tuesday morning in Washington the office of the director of national intelligence, James R. Clapper Jr., issued a statement suggesting the North Koreans were, on their third try, beginning to produce nuclear devices with substantial explosive power. "The explosion yield was approximately several kilotons," the announcement said, which was less specific than a South Korean Defense Ministry estimate of six to seven kilotons.
That would be far greater than the yield of less than one kiloton detected in the North's 2006 test, but it is unclear how it would measure up to the last test, in 2009, which had an estimated yield of two to six kilotons. By comparison, the first bomb the United States dropped on Japan, which devastated Hiroshima in 1945, had an explosive yield of 15 kilotons.
The claim about miniaturizing the device could be important, if true. But there has been no proof yet that the North has yet mastered the difficult technology of making bombs small enough to be fitted to ballistic missiles that could eventually be lobbed as far as the United States mainland.
The test drew a crescendo of international denunciations, with President Obama calling it a "highly provocative act" that demands "swift and credible action by the international community" against North Korea. Russia, Britain, South Korea and the United Nations also quickly condemned the blast. The head of the international nuclear watchdog called the test "deeply regrettable," and the United Nations Security Council -- which has already passed three resolutions aimed at punishing North Korea for its nuclear weapons-related work, met in emergency session to devise a fourth resolution.
Foreign Minister Kim Sung-hwan of South Korea, whose country holds the monthly rotating presidency of the Security Council, emerged from the meeting before noon to read a statement from all 15 members that they had "strongly condemned this test," and were beginning to work immediately on "appropriate measures in a Security Council resolution." He declined to specify what was envisioned but emphasized that all members, including North Korea's ally and neighbor China, wanted action that would convince the North to "abandon its nuclear ambition."
The South Korean foreign minister also said North Korea would "be held responsible for any consequences of this provocative act."
Susan E. Rice, the United States ambassador to the United Nations, told reporters that the Security Council "must and will deliver a swift, credible and strong response." She also declined to specify what a new resolution might do, but said "we and others have a number of further measures that we will be discussing" that would tighten existing measures and "augment the sanctions regime."
Preliminary estimates by South Korea suggested that the test was more powerful than the previous two conducted by the North.
The test is the first under the country's new leader, Kim Jong-un, and an open act of defiance to the Chinese, who had urged Mr. Kim not to risk open confrontation by setting off the weapon. In a relatively muted statement issued several hours after the blast, China expressed its "staunch opposition" to the test but called for "all parties concerned to respond calmly." And it was unclear how China will handle any Security Council push for more sanctions.
The nuclear test came the same day Mr. Obama is to use his State of the Union address to call for drastically reducing nuclear arms around the world, potentially bringing the number of deployed American weapons to roughly 1,000 from the current 1,700.
Even before the North conducted Tuesday's test, the Obama administration had already threatened to take additional action to penalize the country through the United Nations. But the fact is that there are few sanctions left to apply against the most unpredictable country in Asia. The only penalty that would truly hurt the North would be a cutoff of oil and other aid from China. And until now, despite issuing warnings, the Chinese have feared instability and chaos in the North more than its growing nuclear and missile capability.
Mr. Kim, believed to be about 29, appeared to be betting that even a third test would not change the Chinese calculus, and later Tuesday, the North Korean Foreign Ministry warned of "second and third measures of greater intensity" if Washington remains hostile.
The test set off a scramble among Washington and its Asian allies to assess what the North Koreans had done.
The United States sent aloft aircraft equipped with delicate sensors that may, depending on the winds, be able to determine whether it was a plutonium or uranium weapon. The Japanese defense minister, Itsunori Onodera, said Japan had ordered the dispatch of an Air Self-Defense Force jet to monitor for radioactivity in Japanese airspace.
Japan's new prime minister, Shinzo Abe, told Parliament that the country was considering "its own actions, including sanctions, to resolve this and other issues."
But the threat may be largely empty, because trade is limited and the United States and its allies have refrained from a naval blockade of North Korea or other steps that could revive open conflict, which has been avoided on the Korean Peninsula since an armistice was declared 60 years ago.
It may take days or weeks to determine independently if the test was successful. American officials will also be looking for signs of whether the North, for the first time, conducted a test of a uranium weapon, based on a uranium enrichment capability it has been pursuing for a decade. The past two tests used plutonium, reprocessed from one of the country's now-defunct nuclear reactors. While the country has only enough plutonium for a half-dozen or so bombs, it can produce enriched uranium well into the future.
After the detonation, the K.C.N.A. news agency said that the test demonstrated that North Korea's nuclear deterrence has become "diversified." South Korean officials said they were studying whether it meant that North Korea had actually used highly enriched uranium for bomb fuel, rather than plutonium.
No country is more interested in the results of the North's nuclear program, or the Western reaction, than Iran, which is pursuing its own uranium enrichment program. The two countries have long cooperated on missile technology, and many intelligence officials believe they share nuclear knowledge as well, though so far there is no hard evidence. Iran is preparing for two important sets of negotiations with the International Atomic Energy Agency, the United Nations's nuclear regulatory body, starting in on Wednesday, and later this month with the six world powers seeking to curb its nuclear program.
Yukiya Amano, the director general of the I.A.E.A., which is based in Vienna, said in a statement on Tuesday that North Korea's action was "deeply regrettable and is in clear violation of U.N. Security Council resolutions."
He also offered to "contribute to the peaceful resolution" of the North Korean nuclear issue by "resuming its nuclear verification activities in the country as soon as the political agreement is reached among countries concerned."
The timing of the test was critical. It came just as a transition of power is about to take place in South Korea, and the North detested the South's departing president, the hard-line Lee Myung-bak. By conducting a test just before he leaves office, the North could have been sending a message and giving his successor, Park Geun-hye, the chance to restore relations after the breach a test will undoubtedly cause.
The nuclear test came just weeks after the Security Council unanimously passed a resolution calling for the tightening of sanctions against North Korea for a recent rocket launching, a violation of earlier resolutions prohibiting the country from testing ballistic missile technology.
Stung by the promise of stiffer sanctions, the North ratcheted up its threats, vowing to build its capacity to "target" the United States in its most explicit warnings yet. The statement last month, one in a series of threatening statements over several days, said the country planned to test more long-range rockets ("one after another") and to conduct a nuclear test, despite Washington's warning that such actions would lead to more penalties for the impoverished country.
The North has often lashed out when it felt ignored, especially by the United States. It was unclear if the untested Mr. Kim was following a pattern of behavior perfected by his father, the last North Korean leader, in which the North provoked the West and Seoul to win more badly needed aid as an inducement to draw it back to international negotiations on its weapons programs.
Analysts suspect that Mr. Kim, in the face of more sanctions, might have felt a more urgent need to assert his standing among his people, who continue to suffer crippling food shortages they are told is the price of developing a costly and credible deterrence. He also might have needed to improve his standing with the military, which has been considered crucial to keeping the Kims in power, analysts said.
David E. Sanger reported from Washington, and Choe Sang-hun from Seoul, South Korea. Reporting was contributed by Jane Perlez from Beijing, Hiroko Tabuchi from Tokyo, Chris Buckley and Gerry Mullany from Hong Kong, Alan Cowell from Paris, Neil MacFarquhar from the United Nations and Rick Gladstone from New York.
This article originally appeared in The New York Times.