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North Korea on Tuesday staged its most powerful nuclear test yet, claiming a breakthrough with a "miniaturised" device in a striking act of defiance to global powers including its sole patron China.
The communist state said it had staged its third test with a "successful" underground detonation, in a riposte to what it said was US "hostility".
Its claim of miniaturisation suggests that it is a step closer to fitting a nuclear warhead onto a ballistic missile.
The confirmation from North Korean news agency KCNA came nearly three hours after seismic monitors detected an unusual tremor at 0257 GMT in the area of the country's Punggye-ri nuclear test site, close to the Chinese border.
Analysts said the timing appeared to be an attention-grabbing calculation from a state well versed in provocative acts, coming just ahead of US President Barack Obama's State of the Union address at the start of his second term.
North Korea's two previous tests in 2006 and 2009 triggered waves of UN sanctions, and the Security Council was set to meet in emergency session on Tuesday morning in New York in response to the third detonation.
There was no immediate response from US or Chinese leaders, but Beijing had made its displeasure clear to the the youthful Kim Jong-Un's regime in Pyongyang, a UN diplomat said.
"The Chinese gave the North Koreans a strong warning against carrying out a test as it became apparent that it was imminent," said the diplomat, speaking on condition of anonymity, labelling the test "a big challenge to the Chinese".
The test drew condemnation from Japan and UN chief Ban Ki-moon. The United States said only, via the office of the Director of National Intelligence, that its spy agencies were evaluating a "seismic event" in the Stalinist state.
On a technical level, along with the miniaturisation aspect, experts are hungry to know if North Korea used up more of its scarce reserves of plutonium, or exploited uranium in a new and self-sustaining path to atomic detonations.
The test came after North Korea earlier Tuesday had called for "high-intensity" action and further long-range rocket launches, after incurring UN wrath for firing a ballistic rocket in December.
Tuesday's explosion yielded six to seven kilotons, South Korean defence ministry spokesman Kim Min-Seok told reporters, significantly more than the 2006 and 2009 tests.
He said it was unclear yet whether uranium was used.
The explosive yield compared with 15 kilotons in the world's first atomic bomb dropped by the United States on the Japanese city of Hiroshima in 1945.
North Korea's first test yielded less than one kiloton and was widely seen as a dud. The second test yielded between two and six kilotons, according to Seoul.
The third test throws down a stark security and diplomatic challenge to Obama as well as to new Chinese leader Xi Jinping.
Masao Okonogi, an expert on North Korea and professor emeritus at Keio University in Japan, noted the timing ahead of the State of the Union and said North Korea under Kim Jong-Un was sticking to its past tactics.
"Their tactic is to produce a crisis and press the international community to negotiate with them," he told AFP.
"North Korea will not stop just with this nuclear test but continue dragging out this nuclear crisis towards July, when Washington will celebrate the 60th anniversary of the ceasefire in the Korean War.
"North Korea's strategy is to upgrade the ceasefire to a peace treaty. Pyongyang will soon start a campaign, in which it will try to normalise ties with the United States."
Pyongyang's promise of a "higher-level" test had fuelled speculation it would be of a uranium device.
A uranium test would confirm suspicions that the North has been secretly enriching weapons-grade uranium for years and open a path for Pyongyang to significantly expand its small nuclear arsenal.
There will be particular concern at any sign that the North has made progress in the technically complex process of "miniaturizing" a bomb to fit on the head of a long-range missile.
Proven miniaturization ability would take on added significance in the wake of December's rocket launch, which marked a major step forward in ballistic prowess, and provoked still-tighter UN sanctions.
At the UN Security Council, the United States and its allies will push hard for China to get tough with its erratic ally.
But China's leverage is limited, analysts say, by its fear of a North Korean collapse and the prospect of a reunified, US-allied Korea directly on its border.