BEIJING -- 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 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 three and 10 kilotons but added that the South Korean government had produced a preliminary estimate of six to seven 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.