North Korea said Wednesday it had carried out a "successful" miniaturised hydrogen bomb test -- a shock announcement that, if confirmed, would massively raise the stakes in the hermit state’s bid to strengthen its nuclear arsenal
The announcement triggered swift international condemnation but also scepticism, with experts suggesting the apparent yield was far too low for a thermonuclear device which the North was believed to be years from developing.
The United Nations Security Council will hold an emergency meeting Wednesday in New York, diplomats said.
The closed-door morning talks between the 15 member nations were called by the United Nations and Japan.
"The republic's first hydrogen bomb test has been successfully performed at 10:00 am (0130 GMT)," North Korean state television announced.
"With the perfect success of our historic H-bomb, we have joined the rank of advanced nuclear states," it said, adding that the test was of a miniaturised device.
The television showed North Korean leader Kim Jong-Un's signed order -- dated December 15 -- to go ahead with the test, with a handwritten exhortation to begin 2016 with the "thrilling sound of the first hydrogen bomb explosion".
"Let's begin the year of 2016 ... with the thrilling sound of our first hydrogen bomb explosion, so that the whole world will look up to our socialist, nuclear-armed republic and the great Workers' Party of Korea!" Kim wrote in a handwritten message next to his signature.
The television also showed a second order dated January 3 in which Kim signed off his final approval for the test to be conducted on January 6.
South Korean President Park Geun-Hye condemned the test as a "grave provocation" and called for a strong international response as the UN Security Council called an emergency meeting.
The White House said it was still studying the precise nature of the apparent test and vowed to "respond appropriately".
A hydrogen, or thermonuclear, bomb uses fusion in a chain reaction that results in a far more powerful explosion than the fission blast generated by uranium or plutonium alone.
Last month Kim suggested Pyongyang had already developed such a device.
The claim was questioned by international experts at the time and there was continued scepticism over Wednesday's test announcement, which took the entire international community by surprise.
- Scepticism over H-bomb -
"The seismic data that's been received indicates that the explosion is probably significantly below what one would expect from an H-bomb test," said Australian nuclear policy and arms control specialist Crispin Rovere.
The test, which came just two days before Kim Jong-Un's birthday, was initially detected by international seismology centres as a 5.1-magnitude tremor next to the North's main Punggye-ri nuclear test site in the northeast of the country.
Bruce Bennett, a senior defence analyst with the Rand Corporation, said if it was an H-bomb that was tested, then the detonation clearly failed -- at least the fusion stage.
"If it were a real H-bomb, the Richter scale reading should have been about a hundred times more powerful," Bennett told AFP.
Most experts had assumed Pyongyang was years from developing a thermonuclear bomb, while assessments were divided on how far it had gone in developing a miniaturised warhead to fit on a ballistic missile.
Whatever the nature of the device, it was North Korea's fourth nuclear test and marked a striking act of defiance that flew in the face of enemies and allies alike, who have warned Pyongyang it would pay a steep price for moving forward with its nuclear weapons programme.
- Challenge to UN, Obama -
The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions. Their failure to prevent a fourth detonation will put the Security Council under intense pressure to take more drastic action this time around.
It throws down a particular challenge to US President Barack Obama, who, during a visit to South Korea in 2014, lashed North Korea as a "pariah state" and vowed sanctions with "more bite" if Pyongyang went ahead with another test.
White House National Security Council spokesman Ned Price said he could not confirm the H-bomb claim, but promised the US would "respond appropriately to any and all North Korean provocations".
Japanese Prime Minister Shinzo Abe called it a "serious threat" to Japan and a "grave challenge" to nuclear non-proliferation efforts.
The response of China, North Korea's economic and diplomatic patron, will be key. Beijing has restrained US-led allies from stronger action against Pyongyang in the past, but has shown increasing frustration with its refusal to suspend testing.
In an initial reaction the foreign ministry in Beijing said it "firmly opposes" the nuclear test, which was carried out "irrespective of the international community's opposition".
"Beijing will face increased pressure both domestically and internationally to punish and rein in Kim Jong-Un and to ultimately force Pyongyang to give up its nuclear weapons," said Yanmei Xie, the International Crisis Group's senior analyst for Northeast Asia.
- China's fears -
But China's leverage over Pyongyang is restricted by its overriding fear of a North Korean collapse.
"For Beijing, a nuclear-armed North Korea is uncomfortable and disturbing, but a regime collapse in Pyongyang leading to mass chaos next door and potentially a united Korean peninsula with Washington extending its influence northward to China's doorstep is downright frightening," Xie said.
China has been pushing for a resumption of six-party aid-for-disarmament talks on North Korea, insisting that engagement with Pyongyang is the only way forward.
The six-party process, involving the two Koreas, the United States, China, Japan and Russia, has been in limbo since 2008. Pyongyang's decision to move ahead with a fourth test has almost certainly hammered the final nail in its coffin