Pyongyang says it has successfully carried a fifth test of a nuclear device.  The confirmation comes hours after the United States Geological Survey detected a magnitude 5.3 disturbance in the northeast of the country.

The fifth and biggest test so far is in violation of international sanctions.  US President Barack Obama consulted with the leaders of Japan and South Korea and says there will be "serious consequences" for North Korea's defiance.

"The president indicated he would continue to consult our allies and partners in the days ahead to ensure provocative actions from North Korea are met with serious consequences," said a White House spokesman.

China's foreign ministry statement read: "Today, (North Korea) again conducted a nuclear test despite widespread international opposition - the Chinese government firmly opposes the test."

Japanese Prime Minister Shinzo Abe said his country "absolutely cannot condone" any North Korean nuclear weapons tests:  "North Korea's nuclear development is becoming a graver threat to Japan's safety and severely undermines the peace and safety of the region and the international community," said Mr. Abe.

The blast is most-powerful North Korea has attempted, but it is still smaller than the US atomic bombing of Hiroshima towards the end of World War II.  South Korean intelligence believes the latest blast is around twice as strong as previous tests.  Pyongyang claims the test had been of a "newly developed nuclear warhead" and that it was now capable of mounting a nuclear device on ballistic rockets.  North Korea has carried out several missile tests in recent months.