BEIJING: North Korea has once again defied the world to conduct its third nuclear test, taking the hermit kingdom a step closer to a credible nuclear weapon that could theoretically strike the United States or Australia.
A statement from the state-run KCNA news agency confirmed the test, hours after a seismic shock registering about 5 was recorded emanating from the northern region of North Korea, where two nuclear tests were conducted in 2006 and 2009.
Korean military expert: Nuclear test no surprise
Korean military intelligence expert retired Colonel Cedric Leighton says North Korea has set off a larger weapon than in the two previous tests.
''It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturised and lighter nuclear device with greater explosive force than previously, did not pose any negative impact on the surrounding ecological environment,'' the statement said.
The US Geological Survey said the epicentre of the quake was a kilometre underground and the blast took place at 11.57am Korean time.
The test comes amid acute concern in Seoul, Tokyo and Washington that North Korea is about to cross the technology threshold to possessing a credible nuclear warhead and delivery capability.
The UN Security Council was reportedly convening an emergency meeting in New York but potential target nations are likely to seek tougher measures with or without UN backing. The Japanese Prime Minister, Shinzo Abe, immediately convened his country's security council.
''I have ordered we consider every possible way to address this issue, including our own sanctions, while co-operating with other countries,'' he said after the meeting.
The US President, Barack Obama, said the test was a threat to regional stability that undermined UN Security Council resolutions and warranted ''swift and credible'' action by the international community, according to a White House statement.
''North Korea's nuclear weapons and ballistic missile programs constitute a threat to US national security and to international peace and security,'' Mr Obama said.
Vowing ''firm action'', Mr Obama said the US would remain vigilant in the ''face of North Korean provocations'' and stick by its security commitments to allied nations in the region.
He said the US would continue to take the necessary steps to defend itself and its allies.
South Korea, Japan and the US are searching for fresh points of leverage that might convince North Korea's sole ally, neighbouring China, to intervene and obstruct the nuclear weapons program.
There were no signs of a tougher line from China under its new president, Xi Jinping. A spokesman stuck close to previous gentle formulations on Tuesday night.
China expressed ''resolute opposition'' to the test and called on ''all sides'' to respond ''calmly, through talks''.
Last week, the chairman of South Korea's Joint Chiefs of Staff, General Jung Seung-jo, pledged to pre-emptively strike against North Korea if it showed intent to use a nuclear weapon ''even at the risk of war''. Such a strike would hit a sensitive nerve in Chinese security circles, where North Korea is considered a strategically crucial buffer state.
But Chinese regional security analysts seem convinced Beijing could not force Pyongyang to give up its nuclear weapons program even if it tried.
The possibility of a North Korean nuclear-tipped missile reaching as far as Los Angeles or Darwin is no longer considered fanciful after the North successfully tested a ballistic missile in December. The reference to a ''miniaturised'' device in the North Korean statement suggests it has developed a warhead small enough to be fixed to such an intercontinental missile.
The latest test places China in a tighter bind, because the country is now surrounded on all sides by nuclear weapons states.