North Korea on Friday said it had successfully carried out its fifth nuclear test, triggering international condemnation amid concerns about an acceleration in its nuclear capabilities.
North Korea said the test had been of a nuclear warhead that could be mounted on a ballistic missile and that it was now in a position to produce as many "smaller, lighter and diversified" nuclear weapons as it wanted "at will".
"This has definitely put on a higher level the DPRK's technology of mounting nuclear warheads on ballistic rockets," the country's nuclear authority said in an English-language statement on the state-run Korean Central News Agency.
The test was roundly condemned by Japan and South Korea, while the US said Pyongyang faced "serious consequences."
South Korea's Joint Chiefs of Staff said the test was likely to have been North Korea's "biggest-ever," adding that the yield was thought to have been 10 kilotons, South Korea's Yonhap news agency reported.
The bomb dropped by US forces on Hiroshima in 1945 had a yield of 16 kilotons.
The first sign of the test was an earthquake measuring magnitude 5.0 located near North Korea's Punggye-ri underground nuclear test site, where all of the country's tests have been carried out since it began testing in 2006 and where recent satellite imagery had shown fresh activity.
It is thought Friday's test was planned to coincide with the 68th anniversary of the Pyongyang regime, founded by Kim Il Sung and now led by his grandson Kim Jong Un.
South Korea's President Park Geun Hye slammed the test as a provocation that would lead to further international sanctions and said Seoul would use "all possible measures" to force Pyongyang to give up its nuclear programme.
Japan's Chief Cabinet Secretary Yoshihide Suga said North Korea's nuclear programme "seriously undermines" international peace and security and that Tokyo would consider further sanctions.
Japanese Prime Minister Shinzo Abe had earlier also told reporters that a confirmation of the nuclear test, would be "absolutely unacceptable and we would have to lodge a strong protest."
South Korea's National Security Council was also holding an emergency meeting following the test, a spokesman for the president's office said.
White House spokesman Josh Earnest said US President Barack Obama, returning from a trip to Asia, had been briefed on the test and had consulted with Park and Abe.
"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," Earnest said.
China, traditionally Pyongyang's only ally, expressed "firm opposition" to Pyongyang's nuclear test.
"We strongly urge the DPRK to honour its commitment to de-nuclearization, comply with relevant Security Council resolutions, and take action to stop the deterioration of the situation," the Foreign Ministry said.
The head of the UN's nuclear watchdog, Yukiya Amano, called the test "deeply troubling."
Tension has been high since North Korea's fourth nuclear test in January and a controversial rocket launch in February, events which led the United Nations Security Council to tighten sanctions on the isolated nation.
The Council again condemned Pyongyang on Tuesday, a day after it fired three ballistic missiles into the Sea of Japan, coinciding with a G20 summit in the Chinese city of Hangzhou.
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North Korea has carried out its fifth nuclear test since it began its quest for nuclear arms in 2006, causing fresh concerns for its neighbours.