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.
"The republic's first hydrogen bomb test has been successfully performed at 10:00 am (0130 GMT)," North Korean state television announced.
"We have now 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".
South Korean President Park Geun-Hye condemned what she described as a "grave provocation" and called for a strong international response as the UN Security Council planned to hold an emergency meeting later Wednesday.
The North's main ally China voiced its strong opposition, while the White House said it was still studying the precise nature of the 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.
- Experts sceptical -
That claim was questioned by international experts at the time and there was continued scepticism over Wednesday's test announcement, which took the international community by surprise.
"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 as a 5.1-magnitude tremor at the North's main Punggye-ri nuclear test site in the northeast of the country.
The weapons yield was initially estimated at between six and nine kilotons -- similar to the North's last nuclear test in 2013.
The first US hydrogen bomb test in 1952 had a yield of 10 megatons.
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.
South Korea's defence ministry also told reporters it doubted Wednesday's explosion was thermonuclear in nature.
There were expressions of concern but no public panic on the streets of Seoul, where people have become largely inured to North Korea's provocations over the years.
Most experts had assumed Pyongyang was years from developing a hydrogen bomb, while assessments were divided on how far it had gone in developing a miniaturised warhead to fit on a ballistic missile.
- Gesture of defiance -
Whatever the nature of the device, it was North Korea's fourth nuclear test and marked a striking act of defiance in the face of warnings from enemies and allies alike that Pyongyang would pay a steep price for moving forward with its nuclear weapons programme.
The North's official news agency was unrepentant.
US "imperialists" had escalated the situation on the Korean peninsula to the brink of war, defying the North's calls for a peace treaty, it said.
"The more frantic the hostile forces get in their moves to isolate and stifle the DPRK (North Korea), the stronger its nuclear deterrent will grow, bringing them to deathbed repentance."
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, vowed sanctions with "more bite" if Pyongyang went ahead with another test.
The final 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 Pyongyang's 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," said Yanmei Xie, the International Crisis Group's senior analyst for Northeast Asia.
But China's leverage over Pyongyang is restricted by its overriding fear of a North Korean collapse.
"A nuclear-armed North Korea is uncomfortable and disturbing," Xie said.
"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."
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