By Jonathan Marcus
BBC diplomatic correspondent
The seismic evidence is still being analysed, but North Korea's announcement is already prompting shock waves around the region.
The priority now will be to manage the initial consequences of the North Korean announcement - to ensure the understandable tensions do not prompt some sort of military incident and, above all, to frame a firm and coherent diplomatic response.
The North Korean move comes against a chorus of voices - both in Asia and beyond - urging it not to go down this path.
Only last week, the UN Security Council warned that a test would "jeopardise peace, stability and security in the region and beyond".
The focus of any diplomatic response will again be the UN Security Council.
There will be pressure to push for a new resolution condemning the North Korean government and probably demanding a stiff menu of economic sanctions against Pyongyang.
North Korea is of course already one of the most isolated countries in the world.
China remains its critical economic life-line, and while Beijing is far from happy with North Korea's behaviour it will not want to precipitate turmoil in the country that could prompt the collapse of the regime.
But the ramifications of this event go well beyond North Korea.
Arms control experts fear that other countries in the region might now consider developing nuclear weapons programmes of their own.
And if Asia succumbs to a new nuclear arms race the whole flimsy fabric of the nuclear non-proliferation regime could collapse with consequences not just in Asia, but in the Middle East and elsewhere.