North Korea staged an apparent nuclear test of six to seven kilotons Tuesday in a striking act of defiance that, if confirmed, is sure to trigger global condemnation from enemies and allies alike.
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.
North Korea had provided China and the United States with advance warning that a test was imminent, Kim said, after the communist state earlier Tuesday called for "high-intensity" action and further long-range rocket launches.
In Vienna, the Comprehensive Nuclear Test Ban Treaty Organisation said the location of Tuesday's explosion was "roughly congruent" with the 2006 and 2009 tests, and labelled the event a "clear threat" to international peace.
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.
The South Korean defence ministry spokesman said it was unclear yet whether uranium was used.
Regional seismic monitoring agencies detected a seismic event, of a magnitude between 4.9 and 5.1, at 11:57 am (0257 GMT) with the epicentre in the same location as the North's Punggye-ri nuclear test site.
The explosive yield estimated by Seoul's defence ministry 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.
Pyongyang has been threatening a "higher-level" nuclear test for weeks despite warnings of severe repercussions from the UN Security Council.
If confirmed, the third test will throw down a stark security and diplomatic challenge to US President Barack Obama at the start of his second term, hours before he delivers the State of the Union address to Congress.
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 its young leader 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 under new leader Xi Jinping 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.