North Korean television shows leader Kim Jong-Un signing a document allowing for a hydrogen bomb test, in Pyongyang.  Picture: AFP PHOTO
North Korean television shows leader Kim Jong-Un signing a document allowing for a hydrogen bomb test, in Pyongyang. Picture: AFP PHOTO

NEW YORK — The United Nations (UN) Security Council agreed in emergency talks on Wednesday to prepare further measures against North Korea after it carried out a fourth nuclear test.

The 15-member council including China, Pyongyang’s ally, "strongly condemned" the test and described it as a "clear threat to international peace and security".

UN chief Ban Ki-moon said the development was deeply troubling. He unequivocally condemned the underground test and demanded that North Korea "cease any further nuclear activities".

The test announced by North Korea earlier on Wednesday drew swift condemnation from the international community, including the US, which said it was still studying the precise nature of the test and vowed to "respond appropriately."

"We have now joined the rank of advanced nuclear states," North Korean state television said, adding that the test was of a miniaturised device.

State 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.

Beijing has restrained US-led allies from stronger action against Pyongyang in the past, but has shown increasing frustration with the North’s refusal to suspend testing. The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions.

The new test, which came just two days before Mr Kim’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 kilotonnes — similar to the North’s last nuclear test in 2013.

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.

Some sceptics dismissed North Korea’s claim. "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 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 an H-bomb was actually tested, 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," Mr Bennett said.

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.

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 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 final response of China, North Korea’s economic and diplomatic patron, will be key in determining the international community’s next step.

"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."