North Korea on Tuesday announced that it had conducted a nuclear test in what amounted to a sharp challenge of the U.N. Security Council, which warned the rogue nation last month of "significant action" if it undertook such a provocation.
North Korean state media said the nation tested a "miniaturized" nuclear device. If true, the development strongly suggests that Pyongyang is working to develop a nuclear warhead capable of fitting on top of a missile.
In December, Pyongyang had defied United Nations' sanctions by launching a satellite that was seen as a thinly disguised test of ballistic missile technology.
With President Barack Obama scheduled to give his State of the Union address on Tuesday night, it remains unclear what recourse the United States has to address the situation.
The director of the Comprehensive Test Ban Treaty Organization, the Vienna-based U.N. agency that monitors nuclear tests, said that if confirmed the North Korea explosion "would constitute a clear threat to international peace and security."
North Korea's acknowledgement of the test came hours after sensors had detected an artificial earthquake inside North Korea. South Korea reacted by calling for an emergency meeting of the Security Council, which was expected to convene Tuesday morning in New York, but it was not clear what new measures the council might take that would dissuade North Korea from its nuclear program. Past sanctions have neither dismantled the North Korean nuclear program nor curbed the country's behavior.
U.S. and international officials were cautious in their initial reaction to the test reports, with the Office of the Director of National Intelligence in Washington, which oversees U.S. intelligence agencies, saying that it was "aware of a seismic event with explosive characteristics in North Korea and we are evaluating all relevant information."
But James Acton, a physicist and nuclear weapons expert with the Carnegie Endowment for International Peace, a policy institute in Washington, said that 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" _ the third by North Korea since 2006.
"The Secretary-General is gravely concerned about the negative impact of this deeply destabilizing act on regional stability as well as the global efforts for nuclear non-proliferation," the statement said. "He once again urges the DPRK" _ North Korea _ "to reverse course and work towards de-nuclearization of the Korean peninsula."
Based on seismic date from the U.S. Geological Survey, which estimated the tremor caused by the explosion at 5.1 on the Richter scale, Acton said the blast's yield was between 3 and 10 kilotons but added that the South Korean government had produced a preliminary estimate of 6 to 7 kilotons. One kiloton is the equivalent of 1,000 metric tons of TNT. The atomic bomb dropped by the United States on the Japanese city of Nagasaki on Aug. 9, 1945, had a yield of 21 kilotons.
Two key questions about the suspected blast will be what kind of device was used and whether North Korean scientists have been able to produce sufficient amounts of highly enriched uranium from a facility that the secretive Stalinist regime only revealed in 2010.
A statement by the head of CTBTO said the "unusual seismic event" in North Korea "shows explosion-like characteristics."
"Its location is roughly congruent with the 2006 and 2009 (North Korean) nuclear tests," CTBTO Director General Tibor Toth said. "For now, further data and analysis are necessary to establish what kind of event this is. If confirmed as a nuclear test, this act would constitute a clear threat to international peace and security, and challenges efforts made to strengthen global nuclear disarmament and non-proliferation, in particular by ending nuclear testing."
North Korea's 2006 test was confirmed to have involved a device that used plutonium reprocessed from the used fuel rods of a shuttered nuclear reactor. U.S. intelligence officials determined from radioactive debris found in air samples that the yield was less than a kiloton.
A 2009 underground test blast is believed to also have been produced by a plutonium device, although there has been no confirmation, said Acton. It is believed to have been almost as powerful as the Nagasaki bomb, with the Russian Defense Ministry estimating the yield at 20 kilotons.
Acton said that if the latest suspected explosion "vented" radioactive material into the atmosphere, analyses of samples collected by specialized aircraft believed to be operated by the United States and other countries will determine if the device was fueled by highly enriched uranium.
"North Korea has only a limited amount of plutonium," Acton said. "If this test turns out to be highly enriched uranium, that means they have mastered highly enriched uranium production and that means they can make their arsenal much bigger relatively quickly."
North Korea is not thought to have the ability to place a nuclear warhead on top of a missile capable of reaching the United States, but the twin tracks of developing longer-range missiles and conducting further nuclear tests raise the question of whether it's working in that direction.
Tuesday's nuclear test fell in line with threats made last month by North Korea's National Defense Commission to do just that. The commission is headed by the nation's young leader, Kim Jong Un, who's thought to be about 30 years old.
The test will almost certainly strain relations with North Korea's largest backer, China, which has signaled repeatedly that it has tired of diplomatic headaches brought on by the secretive nation.
The Global Times newspaper in Beijing, which while not an official mouthpiece of the Chinese Communist Party is state-controlled, ran a strongly worded editorial earlier this month saying that, "if North Korea insists on a third nuclear test despite attempts to dissuade it, it must pay a heavy price. The assistance it will be able to receive from China should be reduced."
Because of the strategic importance Chinese officials see in maintaining relations with North Korea _ maintaining a buffer between China and U.S. troops stationed in South Korea, and avoiding the chaos of a fallen North Korea spilling over its borders _ Beijing is not likely to be anywhere close to pulling the plug on aid and support.
But frustrations with Pyongyang in China, which is currently in the middle of Chinese New Year celebrations, might well lend support to stronger sanctions by the U.N. Security Council.