North Korea has conducted a "higher level'' nuclear warhead test explosion, which it is trumpeting as finally allowing it to build "at will'' an array of stronger, smaller and lighter nuclear weapons.
It is Pyongyang's fifth atomic test and the second in eight months.
US President Barack Obama says any provocative actions by North Korea will have "serious consequences".
South Korea's president called the detonation, which Seoul estimated had produced the North's biggest-ever explosive yield, an act of "fanatic recklessness.''
The North's boast of a technologically game-changing nuclear test defies both tough international sanctions and long-standing diplomatic pressure to curb its nuclear ambitions.
It will raise serious worries in many world capitals that Pyongyang has moved another step closer to its goal of a nuclear-armed missile that could one day strike the US mainland.
Foreign Minister Murray McCully has also issued a statement calling for a meeting of the UN Security Council, of which New Zealand is currently president.
"This is the fifth nuclear test conducted by North Korea, and it directly defies the UN Security Council which has demanded that North Korea stop nuclear testing," says Mr McCully.
"This latest nuclear test is highly provocative and deeply concerning, especially as it comes on the back of a series of ballistic missile launches by North Korea recently."
Hours after Seoul noted unusual seismic activity near the North's northeastern nuclear test site, Pyongyang said in its state-run media that a test had "finally examined and confirmed the structure and specific features of movement of (a) nuclear warhead that has been standardised to be able to be mounted on strategic ballistic rockets.''
"The standardisation of the nuclear warhead will enable (North Korea) to produce at will and as many as it wants a variety of smaller, lighter and diversified nuclear warheads of higher strike power with a firm hold on the production of various fissile materials and technology for their use. This has definitely put on a higher level (the North's) technology of mounting nuclear warheads on ballistic rockets,'' the country said.
South Korean President Park Geun-hye strongly condemned the test, saying in a statement that it showed the "fanatic recklessness of the Kim Jong Un government as it clings to nuclear development.''
France has also condemned the nuclear test.
"France vigorously condemns the new nuclear test that was conducted last night by North Korea and calls the United Nations' security council to take up this violation of its resolutions," the French presidency said in a statement.
North Korea said there was no radioactive material leaked, but the explosion has put the region on edge.
Chinese state media reported that the nation's environmental protection agency started nuclear radiation monitoring. Japanese planes began to collect air samples from national air space to analyse possible radioactive materials. Tokyo Governor Yuriko Koike said Japan's capital city is also testing water samples and monitoring radiation levels in the air.
The seismic incident comes on the 68th anniversary of the founding of North Korea's government and just days after world leaders gathered in China for the G20 economic summit.
Jeffrey Lewis of the California-based Middlebury Institute of International Studies said the highest estimates of seismic magnitude suggested this was the most powerful nuclear test conducted by North Korea so far.
He said the seismic magnitude and surface level indicated a blast with a 20- to 30-kilotonne yield. Such a yield would make this test larger than the nuclear bomb dropped by the United States on the Japanese city of Hiroshima in World War Two and potentially bigger than that dropped on Nagasaki soon after.
"That's the largest DPRK test to date, 20-30kt, at least. Not a happy day," Lewis told Reuters, using the North's official title of the Democratic People's Republic of Korea.
"The important thing is that five tests in, they now have a lot of nuclear test experience. They aren't a backwards state any more," he said.