The regime's fourth nuclear test - just two days before Kim Jong Un's birthday - could mark a significant enhancement of its military capabilities and came as a surprise to many. Julian Ryall reports from Tokyo.
North Korea conducted its latest underground nuclear test at 10 am local time on Wednesday, January 6, with state media claiming the device was a "miniaturized" hydrogen bomb detonated on the personal orders of Kim Jong Un just two days before his birthday.
The explosion was initially recorded as an earthquake with a 5.1 magnitude by seismic monitoring stations around the world, but analysts and governments were quick to reach the conclusion that Pyongyang had carried out another nuclear test, its first since February 2013.
The biggest clue as to the provenance of the detonation was its location, in the far north-east of the country at the regime's Punggye-ri nuclear proving grounds (shown in the graph below). Less than three hours later, state television announced the test, with a presenter in a "special announcement" telling viewers, "We have now become a nuclear state that also holds a hydrogen bomb."
The broadcast was followed by "spontaneous celebrations erupting in Pyongyang's streets as joyful workers cheer latest developments in DPRK science," the DPRK News Service claimed.
Crisis meeting in Seoul
South Korea reacted by stepping up monitoring of developments in the North, while President Park Geun-hye summoned a meeting of the National Security Council. Park said at the start of the meeting that the government "must get North Korea to face corresponding measures based on closed cooperation with the international community.
"It's not only grave provocation of our national security, but also an act that threatens our lives and future. It's also a direct challenge to world peace and stability," she was quoted as saying.
In Tokyo, Prime Minister Shinzo Abe was quick to react, telling reporters that the North's actions are a "grave challenge" to non-proliferation efforts and that Japan "absolutely cannot tolerate North Korea's nuclear tests." In Washington, officials said they are looking into the reports, while the UN Security Council will meet in New York to discuss the matter.
China said it would lodge a protest with North Korea, adding it did not have advance knowledge of North Korea's test of a miniaturized hydrogen nuclear device.
There had been hints, however, in recent weeks that the North was making strides in its development of a thermonuclear weapon, with Kim announcing on December 10 that his regime is now a "powerful nuclear weapons state ready to detonate self-reliant A-bombs and H-bombs."
That claim, however, was downplayed by the United States, which said there was no evidence to support Mr Kim's claim, although a White House spokesman conceded, "We take very seriously the risk and the threat that is posed by the North Korean regime in their ambitions to develop a nuclear weapon."
Nonetheless, three days before the latest incident, South Korean news agency Yonhap had referred to a report published by the South's Chemical, Biological and Radiological (CBR) Defense Command, which stated the North had already laid the groundwork to develop thermonuclear weapons and may already be producing tritium, a radioactive isotope necessary to build more sophisticated nuclear weapons.
"We can't discount the possibility that the North's excavation of a new tunnel at its Punggye-ri test site could be designed for thermonuclear weapons tests," the command, under the direct control of the Defense Ministry, was quoted as saying.
"Considering its research of nuclear technology, its history of underground and projectile tests, and elapsed time since its nuclear development, North Korea has the foundation for thermonuclear weapons."
Bearing in mind the claims of the regime, experts around the world are scrambling to determine precisely what North Korean scientists have achieved, with Lance Gatling, a defense analyst and president of Nexial Research Inc., telling DW that governments will be using aircraft and ground stations to gather air samples to determine what has taken place.
'A matter of serious concern'
"Any time that a country with a political track record like North Korea's tests a nuclear device or attempts to increase the efficiency and reliability of its nuclear weapons - and, even more worryingly, the deliverability of those weapons - then that is a matter of serious concern," he said.
The belief among many experts, including Gatling, is that the device tested today is not the "pure" H-bomb that North Korea claims, but a weapon that utilizes fusion fuels, such as deuterium or lithium, to dramatically boost the yield of a conventional fission explosion.
The three previous tests show that North Korean scientists have already mastered basic nuclear explosions, but making the leap to a thermonuclear device would potentially give North Korea a weapon with a yield of 100 kilotons, around seven times the power of the bomb that was dropped on Hiroshima in 1945.
"The advances may not be so technologically significant," Gatling said. "But clearly they are making advances and the biggest concern has to be where they are obtaining the material for these devices."
Robert Dujarric, director of the Institute of Contemporary Asian Studies at the Japan campus of Temple University, points out that North Korea is already under international sanctions over its previous nuclear and missile tests and he suggests there is little additional pressure that the rest of the world can bring to bear on the regime to encourage it to desist from such provocative actions.
Last April, Joel Wit, the founder of 38 North at the US-Korea Institute at Johns Hopkins School of Advanced International Studies, told DW that the East Asian country was on the verge of rapidly increasing its nuclear weapons stockpile to 20, 50 or 100 bombs within five years, from an estimated 10-16 weapons at that time, adding to regional concerns.
However, given the opacity of the regime's nuclear program, it is very difficult to credibly assess where the country's strategic weapon programs stand at any particular moment, so even basic questions about its nuclear weapons capabilities are shrouded in considerable uncertainty.
For example, there is no public information to verify how many operational nuclear weapons the country might possess - or indeed, whether it even has produced operational nuclear weapons as opposed to rudimentary nuclear explosive devices, Shannon Kile, nuclear arms control and non-proliferation expert at the Stockholm International Peace Research Institute (SIPRI), told DW.
Development to continue
"It seems to me that the regime truly believes that it needs nuclear weapons to protect itself from the outside world and, however misguided that notion is, they will continue to invest in these sorts of weapons," Kile told DW.
"But further than that, the international community cannot really enforce its will on Pyongyang," he said. "We have seen these sorts of actions for 20 years or more and there are all sorts of sanctions in place, but still they continue.
The explosion was initially recorded as an earthquake with a 5.1 magnitude by seismic monitoring stations around the world
"I expect the rest of the world to condemn this new test and demand that it never happens again, but not do much more because they really can't," he said. "The North will demand talks and concessions - perhaps funds from the South - in return for promises to not do it again, and that will be the end of the matter.
"For the North, this is essentially a risk-free endeavor because there are basically no penalties," he added. "China does not want regime collapse and all the chaos that would go with that; the South is of the same mind and wants to avoid conflict; President [Barack] Obama does not want to commit troops to another war; and Japan is not in the business of invading anyone.
"My sense it that the rest of the world is going to effectively shrug its shoulders after expressing its anger."