"The Chinese side vehemently demands North Korea abides by its denuclearisation promises, stop any actions which may worsen the situation and return to the six-party talks process.
"The Chinese government calls on all sides to calmly and appropriately deal [with the situation]."
'Threat to region'
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China is the closest nuclear superpower. As a member of the non-proliferation treaty, it has undertaken to limit the spread of nuclear weapons, but Pyongyang's latest move could spur it to resume its own nuclear testing
South Korea has a civilian nuclear program to produce electricity, but the border with its northern neighbour is the most heavily militarised area in the world, and the latest nuclear test has considerably raised tensions
Japan is also a civilian nuclear power. As the only country ever attacked with atomic weapons it remains deeply opposed to nuclear arms. Analysts fear that if Japan felt pressured into developing nuclear weapons, it would trigger an arms race across the region
Timeline: North Korea's
nuclear programme
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South Korea called the underground test a "serious threat" to stability in the region, following a meeting between Lee Myung Bak, the president, and senior security officials.
The test posed "a serious challenge to the international regime on nuclear non-proliferation," a statement from the government said.
Seoul said it would work with members of the six-party talks - the US, Japan, China and Russia - on North Korea’s nuclear programme so that the United Nations Security Council could takes "appropriate steps", the statement said.
Japan said that the North's move marked a clear violation of UN Security Council resolutions.
"We will definitely not tolerate it," Takeo Kawamura, Japan's chief cabinet secretary, told reporters in Tokyo.
Arms race
Ron Huisken, a nuclear non-proliferation expert at the Australian National University, said that the test showed Pyongyang remains determined to pursue a nuclear weapons programme, but it did not present an immediate danger to countries in the Asia-Pacific region.
"North Korea can't actually do anything at this point," he told Al Jazeera.
"To the best of our knowledge, it hasn't actually weaponised its nuclear material. Certainly it hasn't miniaturised it to the point where you can put a bomb on an airplane or – even more technically demanding – on top of a missile."
Donald Kirk, a Korea expert based in the South Korean capital, Seoul, said the test was intended to fortify "North Korea's claim to be the ninth nuclear power".
The test "raises the spectre of a nuclear arms race in northeast Asia", which could tempt Japan, Taiwan and South Korea, among others, into developing their own nuclear weapons deterrent, he said.
'Defending sovereignty'
Announcing the test, North Korean state media said the explosion had increased the power of its nuclear weapons arsenal.
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| The test site is believed to be near the northeastern town of Kilchu |
"We have successfully conducted another nuclear test on May 25 as part of the republic's measures to strengthen its nuclear deterrent," the official KCNA news agency said, referring to North Korea's first nuclear test in October 2006.
It said the test would "contribute to defending the sovereignty of the country and the nation and socialism and ensuring peace and security on the Korean Peninsula."
Media in South Korea reported that North Korea also test-fired three short-range missiles shortly after the nuclear blast.