World leaders gathered in New York City today to discuss North Korea’s latest nuclear testing. We talk to Colum Lynch via Skype from the UN about what might come of the United Nations Security Council meeting. (The Fold/The Washington Post)

The North Korean underground nuclear test confirmed by U.S. intelligence agencies on Tuesday served as a stark reminder that the unpredictable and largely inscrutable government in Pyongyang remains a wild card for President Obama’s second term — a nuclear threat to U.S. allies in Asia and a potential arms merchant to the highest bidder.

The timing of the test was interpreted in Washington as an attempt by North Korea’s young new leader to upstage Obama before his State of the Union address. And the claim that it involved a smaller, lighter device — an important element of any deliverable weapon — suggested that the demonstration could be the most dangerous yet by North Korea.

In an unusual pre-dawn statement, Obama called the nuclear test, Pyongyang’s third, a “highly provocative act” that undermines stability in Asia and fails to strengthen North Korea’s own security. Later, in his State of the Union address, he warned North Korean officials that the test “will only isolate them further, as we stand by our allies, strengthen our own missile defense and lead the world in taking firm action in response to these threats.”

Short of the threat of military action, however, the United States and the U.N. Security Council have little leverage over North Korea. Stringent economic sanctions have not halted the North’s nuclear development or alleged proliferation.

The North’s military-backed dictatorship, heavily armed and diplomatically and economically isolated from all but its patron and neighbor China, and its leader, Kim Jong Eun, have rebuffed all recent U.S. efforts to negotiate over its nuclear program.

After a hastily convened emergency session Tuesday, the Security Council branded the nuclear detonation a “grave threat” to world peace and pledged to immediately seek additional binding sanctions against Pyongyang. U.N. Secretary General Ban Ki-moon, a former South Korean foreign minister, also denounced North Korea as the only country to carry out a nuclear test in the 21st century.

All eyes on China

The statement by the 15-nation council set the stage for another high-level U.S.-led effort to persuade veto-holding China to support tougher sanctions.

Western governments were hopeful that Pyongyang’s open defiance of its powerful benefactor in Beijing would lead China to approve fresh penalties. But China is not expected to cut off the lifeline of money, energy assistance and political support that keeps North Korea afloat. Chinese authorities are worried about North Korea’s nuclear ambitions, but they are more worried about a tide of refugees and a security vacuum on its borders if the North implodes.

China issued a statement reiterating its call that North Korea not take “any further actions that would worsen the situation” and cautioning Western powers not to overreact.

The nuclear test comes about two months after the North launched a satellite into space in violation of U.N. resolutions and just weeks after the Security Council adopted a resolution expanding the list of North Korean individuals and companies subject to U.N. sanctions.

Before the Security Council meeting on Tuesday, Susan E. Rice, the U.S. ambassador to the United Nations, who negotiated that resolution with the Chinese, sounded an exasperated note as she prepared for a new round of negotiations. “We’ll do the usual drill,” she said.

“The Security Council must and will deliver a swift, credible and strong response,” Rice said later. She called for a binding resolution that would make it harder for North Korea to pursue its nuclear and ballistic missile programs.

Timeline: Highs and lows in the relationship between North and South Korea

North Korea has found ways around the import bans on materials and technology that could be misused.

Search for clues

As diplomats piled on the criticism, intelligence officials and weapons analysts stepped up their search for clues about the technical significance of North Korea’s third nuclear test since 2006. Although seismic readings suggested that the bomb’s explosive yield was relatively small — less than half that of the atomic bomb dropped on Hiroshima in 1945 — other details about the test may not be known for days or weeks.

A key question was whether North Korea had exploded a plutonium bomb, as it apparently did in 2006 and 2009, or had acquired the capability to make a device using highly enriched uranium, or HEU. Enriched uranium poses a bigger risk for proliferation, because cash-strapped Pyongyang could be tempted to share the fuel with enemies of the West.

North Korea also could assemble a large arsenal using enriched uranium, which it is making indigenously.

“The significance is that North Korea’s stockpile of plutonium is limited — maybe enough for a dozen” devices, including test models, said James M. Acton, a physicist and nuclear expert at the Carnegie Endowment for International Peace. “Thus, the arsenal is limited. But if it has mastered highly enriched uranium, North Korea has a route by which it can expand its arsenal pretty rapidly.”

South Korean officials had speculated that Pyongyang’s next bomb would contain HEU, and North Korean media hinted at such a shift in official statements declaring that the country had achieved a “diversified” nuclear deterrent.

The truth may not be known unless radioactive traces from the test are picked up by monitoring stations near the border or specialized aircraft that sample the air after a nuclear test.

Although North Korea regularly menaces neighbors and U.S. allies South Korea and Japan, nuclear security experts say the greater danger is that the North could sell weapons or uranium to other governments or to terrorists.

The United States is treaty-bound to defend South Korea and Japan, with nuclear weapons if necessary. The prospect of global reductions in weapons stockpiles makes those and other U.S. allies nervous, despite the security gain from having fewer weapons overall.

Obama assured South Korean President Lee Myung-bak on Tuesday that “the United States remains steadfast in its defense commitments to the Republic of Korea, including the extended deterrence offered by the U.S. nuclear umbrella.”

Lynch reported from the United Nations. Joby Warrick contributed to this report.