Details of the blast were sketchy. Sergei Ivanov, the Russian Defence Minister, estimated its yield at between five and 15 kilotons, or up to 15,000 tonnes of TNT, making it potentially as powerful as the bomb which devastated the Japanese city of Hiroshima in 1945.
But an Australian seismology institute put the figure at one kiloton and the Korea Institute of Geoscience and Mineral Resources said the size of the tremor suggested an explosive force equivalent to 550 tons of TNT - a surprisingly low explosive yield.
"If it was a one kiloton explosion, that would be disappointing small - a completely damp squib. It's very unlikely that they would have designed a bomb to have a yield as small as that," said James Acton, science and technology researcher at Vertic, a non-governmental organisation in the UK that works for nuclear non-proliferation.
The blast makes North Korea the eighth country in the world to openly carry out a nuclear test after the United States, Russia, Britain, France, China, India and Pakistan.