North Korea has successfully detonated a miniaturised nuclear device at a northeastern test site, state media says.
Official media said the test was conducted in a safe manner and is aimed at coping with "outrageous" US hostility that "violently" undermines the North's peaceful, sovereign rights to launch satellites.
The announcement came after seismic activity measuring 4.9 magnitude was registered on Tuesday by the US Geological Survey.
UN Secretary-General Ban Ki-moon condemned the test, saying it was "deplorable" that Pyongyang had defied international appeals to refrain from such provocative acts.
"The secretary-general condemns the underground nuclear weapon test conducted by [North Korea] today," Ban's spokesman Martin Nesirky said in a statement. "It is a clear and grave violation of the relevant Security Council resolutions."
South Korean defence ministry spokesman Kim Min-seok, citing seismic data, said the test had an explosive yield of at least six or seven kilotons, significantly more than North Korea's two previous tests in 2006 and 2009.
He also said North Korea had informed China and the United States of its plans to conduct a nuclear test. The South's official news agency said the warning had been given on Monday to both capitals.
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