After an emergency session in New York, Susan Rice, America's ambassador to the UN, told reporters: "To address the persisting danger posed by North Korea's threatening activities, the Security Council must and will deliver a swift, credible and strong response." She insisted that Pyongyang's defiance of existing resolutions banning such tests "will not be tolerated".
The day that ended in fury at the UN began with an explosion one mile below ground that registered with seismologists in South Korea, Japan and the US. At 11.58 Tokyo time, the Japan Meteorological Agency – one of the world's key monitoring stations for North Korea – detected what it carefully described as an "artificial" quake. It was one of the first signs anywhere on the planet that Pyongyang had gone ahead with its long-threatened test.
A minute after the North's test, the agency's sensors were able to tell scientists that the speed of the seismic waves meant the magnitude 5.2 quake was man-made. The data indicated the size of the controlled explosion at about six to seven kilotons, larger than Pyongyang's two previous tests but smaller than the 20-kiloton bomb that obliterated Hiroshima in 1945.
Roughly 13 minutes later, the Agency's bureaucrats informed the Prime Minister's office. A statement by the Prime Minister, Shinzo Abe, was waiting to roll to the world's media, entirely predictable in its outrage. "This nuclear test by North Korea is totally unacceptable as it constitutes a grave threat to Japan's security... and seriously undermines the peace and security of North-east Asia," it said. There was, however, a sting in the tail: Tokyo, he said, would now prevent "in principle" all pro-Pyongyang officials from re-entering Japan.
South Korea's president-elect issued a statement through a spokesman that described the test as "a grave threat to the Korean peninsula and international peace". The provocative act "hampers inter-Korean trust-building and undermines efforts for peace," it added.
The test was a demonstration that North Korea's new leader ,Kim Jong-un, remains determined to carry forward the legacy of his father in developing his country's nuclear arms programme.
He has also inherited his father's belligerence. An official statement said it was a "first response" to perceived US threats and that North Korea would proceed with "second and third measures of greater intensity" if US hostility persists.
Experts from all 15 Security Council members began consultations on a new resolution last night. Typically, it would take the UN weeks to draft a final text but diplomats said this time things should move faster. Much will depend on China's willingness to contemplate more than a symbolic wrist-slap, which has been its default position in the past. Most crippling for Pyongyang would be a shut-off of oil supplies from its long-time ally to the north.
Sources in New York said China had remained coy as to what it might accept in a new sanctions resolution. But it summoned North Korea's ambassador to the Foreign Ministry in Beijing for a dressing down. China was "strongly dissatisfied and resolutely opposed" to the test, its Foreign Minister, Yang Jiechi, said, urging North Korea to "stop any rhetoric or acts that could worsen situations".
Analysts scrambled to discern exactly what North Korea had achieved. If, as Pyongyang claimed, it had managed to produce a larger blast than in 2006 and 2009 from a miniaturised device, it would take North Korea closer to being able to attach a nuclear warhead to its ballistic missiles.
Its past two tests have involved plutonium devices but if it can now harness uranium it would represent a significant technological breakthrough. The process of distilling natural uranium ore for use in weapons is extremely difficult.
President Obama said that the "danger posed by North Korea's threatening activities warrants further swift and credible action by the international community.
"The United States will also continue to take steps necessary to defend ourselves and our allies". He warned that nuclear tests "do not make North Korea more secure".
Q&A: Could Kim Jong-un launch a nuclear strike?
Q. Where was the test carried out?
A. The test, which experts thought at first looked like an earthquake of between 4.7 and 5.2 magnitude, has been pinpointed to the Punggye-ri facility in the north-east, where smaller tests were conducted in 2006 and 2009. Around 12 miles east of Punggye-ri is the Hwasong Gulag – known as Camp 16 – a concentration camp for political prisoners. "Prisoners are never released from Camp 16, dead or alive," says Joshua Stanton, who runs the One Free Korea blog.
Q. What do they hope to achieve?
A. Pyonyang's nuclear capability is what keeps the country in splendid isolation, but also making it a pariah. Until now China has always been supportive but new leader Xi Jinping joined the condemnation. Even Iran, which is suspected of sharing nuclear secrets with Pyong-yang, was critical – although that may be to further its own ends.
Q. Can they launch a nuclear strike?
A. The significance of this test is its strength – and the claim that the device was miniaturised. Previous tests have involved plutonium-based weapons, but this appears to have been a uranium-powered detonation. North Korea possesses ballistic missiles but has never had the capacity to attach a nuclear warhead to them. If a miniaturised detonation is confirmed, this makes it possible to strike at another country.
Q. Are there any sanctions left?
A. The people are among the world's poorest. The only thing left is oil imports.