 "North Korea carries out third nuclear test")
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 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.
Obama denounced the test and called for a "swift" and "credible" international response, as the Security Council readied to meet. Condemnation came from others including Russia, while Japan saw a "grave threat" to its own security.
It was the North's first nuclear test since its new, youthful leader Kim Jong-Un took over from his father, Kim Jong-Il. Security analysts said it sent a bold message of intent following a successful long-range rocket launch in December.
"The launch and the test are empirical evidence that Kim and his regime have no intention of negotiating away the North's nuclear programme any time soon," said Paul Carroll, programme director at the US-based Ploughshares Fund.
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 this morning in New York in response to the third detonation.
There was no immediate response from China, but a UN diplomat said Beijing had made its displeasure clear to Pyongyang.
"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.