Radioactivity is found from 'nuclear test that failed'
However, several other tests carried out by US scientists have failed to determine whether an explosion took place.
And a senior US official said: "The betting is that this was an attempt at a nuclear test that failed.
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Hide Ad"We don't think they were trying to fake a nuclear test, but it may have been a nuclear fizzle."
Meanwhile, the United Nations General Assembly has confirmed that South Korea's foreign minister Ban Ki-moon will succeed Kofi Annan as its next secretary-general.
He immediately made peace with North Korea a top priority.
Mr Ki-moon said: "My tenure will be marked by ceaseless efforts to build bridges and close divides. Leadership of harmony not division, by example not instruction, has served me well so far. I intend to stay the course as secretary-general."
Members of the UN Security Council were to vote today on sanctions to be imposed on North Korea.
The draft resolution authorises non-military sanctions but says that any further action would require another UN resolution.