The nuclear test comes against a backdrop of uncertainty about North Korea’s reclusive leader, Kim Jong-il, and increased speculation about who might succeed him. Mr. Kim suffered a stroke last August, which prompted him to step up preparations to transfer power to one of his three known sons. Analysts believe the favorite son is his youngest, Kim Jong-un, who is in his mid-20s.

North Korea conducted its first nuclear test on Oct. 9, 2006, which was considered something of a bust by South Korean and American officials. North Korea had given some advance notice before that test, which, like Monday’s test, also was conducted in the country’s northeast.

Pyongyang had recently threatened to conduct a second nuclear test, citing what it called Washington’s “hostilities.”

If the North’s latest test was more of a success, it could mean that North Korea has bolstered its atomic weapons capabilities  and its leverage over the United States, which has sought to denuclearize the North.

Image This undated picture, released by the North Korean official news agency on May 23, shows North Korea's leader, Kim Jong-il, inspecting an air force unit. Credit... KNS/Agence France-Presse — Getty Images

In Washington, the Obama administration had a cautious initial reaction to the news early Monday. “We are aware of the reports of a nuclear test by North Korea,” said a senior State Department official who spoke on condition of anonymity because the department was still gathering information. “We are consulting with our allies. Once we have established the facts, we will have more to say.”