Despite recent warnings from the United Sates ,UN and the rest of the world,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".
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 this 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.
Seismic readings from the area around North Korea's nuclear test site detected a "suspected explosion", according to China's Earthquake Administration, and South Korea said all the signs pointed to a test.
"We suspect North Korea has pushed through with a third nuclear test," South Korean defence ministry spokesman Kim Min-Seok told reporters, putting the yield at significantly more than North Korea's two previous tests in 2006 and 2009.
South Korea defence ministry said Earthquake is a sign of a widely-anticipated nuclear test.
North Korea had provided China and the United States with advance warning that a test was imminent, Kim said, after the communist state earlier today called for "high-intensity" action and further long-range rocket launches.
The two previous tests 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 apparent third detonation.
The response of China, North Korea's economic and diplomatic patron, will be key. While restraining US-led allies from stronger action against Pyongyang in the past,Beijing had pressed the country to hold off on the third test.
The apparent test came despite a "strong warning" from China to the North, a Security Council diplomat said. For nuclear experts, the key question will be whether North Korea used up more of its scarce reserves of plutonium for the suspected third test, or used uranium in a new and self-sustaining path to atomic detonations.
Currently , south Korea holds the presidentship of the 15-country Security Council and had been calling for strong action against its arch-rival neighbor in the event of a nuclear test.
The US Geological Survey noted the seismic activity --also confirmed by monitors in South Korea, China and Japan --had a very shallow depth of just one kilometre (0.6 miles). The South Korean defence ministry and the presidential Blue House both said they were trying to verify whether a nuclear detonation had taken place.
It would throw down a stark security and diplomatic challenge to US President Barack Obama at the start of his second term, and to regional neighbours China, Japan and South Korea, all of which have new or incoming leaders.
The first priority for the international community will be determining the precise nature and yield of any test and what it reveals about the technical level of the North's nuclear weapons programme.
Pyongyang's promise of a "higher-level" test had fuelled speculation it would be of a uranium device, compared to the plutonium ones detonated in 2006 and 2009. 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.
Some experts had suggested a simultaneous test of both a plutonium and a uranium device. Even with sophisticated seismic monitoring and "sniffer" planes capable of detecting radioactive fallout, external analysis will provide only limited information on the test,especially if it was well-contained.
There will be particular concern at any sign that the North has made progress in the technically complex process of "miniaturising" a bomb to fit on the head of a long-range missile.