The test is the first under the country’s new leader, Kim Jong-un, and an open act of defiance to the Chinese, who urged the young leader not to risk open confrontation by setting off the weapon. In the past few days a Chinese newspaper that is often reflective of the government’s thinking said the North would “pay a heavy price” if it proceeded with the test. But it was unclear how China would act at the United Nations Security Council, which scheduled an emergency session for Tuesday morning as news of the blast played out.

The United Nations secretary-general Ban Ki-moon, condemned the test in a statement Tuesday.

The Obama administration has already threatened to take additional action to penalize the North through the United Nations in the event of a test. 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, appears to be betting that even a third test would not change the Chinese calculus.

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.