North Korea says it has successfully carried out its fifth nuclear test.
The announcement on state media came hours after a seismic event was detected near its nuclear test site.
South Korea believes it is the North's biggest-ever test, raising fears it has made significant nuclear advances.
South Korean President Park Geun-hye called it an act of "self-destruction" showing the "maniacal recklessness" of leader Kim Jong-un. The US warned of "serious consequences".
China's foreign ministry said Beijing was resolutely opposed to the test and urged North Korea to avoid further action that would worsen the situation.
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The North said the test had been of a "newly developed nuclear warhead" and that it was now capable of mounting a nuclear device on ballistic rockets.
Estimates of the explosive yield of the latest blast have varied. South Korea's military said it was about 10 kilotonnes, enough to make it the North's "strongest nuclear test ever". Other experts say initial indications suggest 20 kilotonnes or more.
The bomb dropped by the US on Hiroshima in 1945 had a yield of about 15 kilotonnes.
Ms Park, who is cutting short an overseas visit, said the test was a "grave challenge" to the international community that would "only earn more sanctions and isolation" for North Korea.
"Such provocation will further accelerate its path to self-destruction," she said.
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A North Korean TV presenter, watched here in the South, reads out the news
Japan's Prime Minister Shinzo Abe said his country "absolutely cannot condone" any such test and would "protest adamantly" to Pyongyang.
"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," he said.
The White House said President Barack Obama had spoken to both Ms Park and Mr Abe after the latest test.
A statement from press secretary Josh Earnest said Mr Obama had "reiterated the unbreakable US commitment to the security of our allies in Asia and around the world".
Analysis: Steve Evans, BBC News, Seoul
What North Korea's opponents can actually do is problematic.
After the fourth test back in January, China agreed to impose tougher UN sanctions. Further and even tougher sanctions are still possible, like blocking the export of fuel oil to North Korea.
That would be a drastic step which might halt the economy and cause serious suffering to ordinary people.
China and the US are increasingly at odds as China asserts itself in East Asia.
China's bottom line is that it does not want the collapse of the regime in Pyongyang if that leads to a chaotic power vacuum, possibly filled by the US and its allies.
"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."
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."
Yukiya Amano, head of the International Atomic Energy Agency, said the test was "in clear violation of numerous UN Security Council resolutions and in complete disregard of the repeated demands of the international community. It is a deeply troubling and regrettable act".
Long-expected test
The test was first detected as a 5.3 magnitude earthquake on Friday morning in north-east North Korea, close to its Punggye-ri underground nuclear test site.
As with previous nuclear tests, the waveform generated indicated it had not been naturally occurring.
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The last test in January had a yield of about 6-8 kilotonnes. Pyongyang said it was of a hydrogen bomb, which employs nuclear fusion, although many analysts cast doubt on that claim.
A fifth test has long been expected. In recent weeks, satellite imagery has shown increased activity at Punggye-ri.
The North also often uses nationally important dates as an opportunity for a show of military strength. Friday is its National Day, celebrating the founding of the current regime.
It is likely to be some time before the scale and manner of the test are independently confirmed.
Japan has despatched military aircraft to collect air samples to monitor for radiation, while China said it was monitoring radiation levels close to its borders with the North.
Missile tests and angry rhetoric
North Korea is banned by UN sanctions from any tests of nuclear or missile technology.
But in recent months it has conducted a series of ballistic missile launches - some of which reached Japanese waters - and has unleashed a rising tide of aggressive rhetoric, threatening nuclear attacks on its enemies.
The North has also been angered by a US and South Korean plan to install an anti-missile defence system in the South and by the allies' massive annual joint military exercises, which are still taking place.
International sanctions were considerably toughened in response to previous nuclear and missile tests but had little impact on Pyongyang's nuclear arms programme.
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