There have been expressions of international outrage after North Korea said it had successfully carried out an underground nuclear test
As seismologists reported an event with the power of a 4.5 quake, US President Barack Obama said North Korea's action threatened "international peace".
A number of external agencies confirmed an explosion, probably associated with a nuclear test, had taken place.
It is being reported that North Korea gave warning of an imminent test to the US less than an hour before it happened.
It appears to have been a much more powerful blast than North Korea's first nuclear test, in October 2006.
An emergency session of the UN Security Council is being convened by Russia, which currently occupies the council's rotating presidency.
BBC world affairs correspondent David Loyn says North Korea appears to have moved from a posture of negotiation to confrontation over the nuclear issue.