North Korea Prepares for "The Fire"

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 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 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 much 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 would act at the Security Council meeting on Tuesday.

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, and the Chinese leadership has refused to participate in sanctions.

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’s 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.”