State Department spokeswoman Victoria Nuland said in Washington that China would be crucial in sending a message to North Korea.

“The Chinese have the most influence. . . . That’s obvious, given their well-intermeshed economic relationship with the DPRK,” Nuland said, using the initials for North Korea’s official name, the Democratic People’s Republic of Korea. “That’s why, among other reasons, it’s so important for us to stay closely linked up with China.”

But whether China would take a tougher stance – moving to cut off oil supplies, for example – was uncertain. Maintaining the status quo in North Korea is important to China, not only because it provides a buffer between China and U.S. soldiers in South Korea, but also because it prevents the chaos of a destabilized nation spilling over its borders.

“It’s unrealistic for us to expect a change overnight,” Zhu Feng, a noted expert on international relations at Peking University, said in a telephone interview Tuesday night. But this most recent round of trouble, Zhu said, could well prove an impetus for Beijing to reconsider its policy.

It also remained far from clear what Pyongyang ultimately hoped to accomplish. North Korea has long been characterized by erratic and secretive actions, and Kim, who’s thought to be about 30 years old, might be fueling the standoff to solidify his domestic standing. He was named the nation’s ruler in December of 2011 after the death of Kim Jong Il, and some observers wondered whether the son was up to the task of succeeding his father.

In the past, North Korea also has used intensely provocative behavior to gain attention on the global stage before making the case for more aid to the impoverished nation.

There were hints of that history in Tuesday’s statement from North Korea. Mixed into the bombastic language about new North Korean actions was another option: The United States could “open a phase of detente and stability.” North Korea tied that scenario to American acceptance of Pyongyang’s “right to satellite launch,” a reference to a launch in December that was widely considered a test of its missile program.

Adding to the mystery was that North Korea had notified the United States that the test was coming. The United States attempted to persuade the North Koreans not to proceed, spokeswoman Nuland said, without success.

Nuland declined to specify how much advance notification the North Koreans had provided, saying only that the contact was made recently.

International nuclear experts were trying Tuesday to determine just what North Korea had exploded and what it would say about the country’s nuclear capabilities.

South Korea’s National Intelligence Service told members of the nation’s intelligence committee that “it is too early to say the North has succeeded in weaponizing its nuclear technology,” according to that country’s Yonhap news agency.

The Office of the Director of National Intelligence in Washington was circumspect in its comment on the test. "North Korea probably conducted an underground nuclear explosion" with a yield of "approximately several kilotons," it said in a statement. "Analysis of the event continues."

James Acton, a physicist and nuclear weapons expert with the Carnegie Endowment for International Peace, a policy institute in Washington, said there was “very little doubt in my mind that this was a nuclear explosion,” and the office of U.N. Secretary-General Ban Ki-moon issued a statement referring to the event as an “underground nuclear weapon test.”