North Korea has successfully tested a nuclear warhead, it said Friday, drawing condemnation from the South which said the "maniacal recklessness" of young ruler Kim Jong-Un would lead to self-destruction.
Pyongyang's state media said the test, which comes after a series of ballistic missile launches that have drawn international condemnation and UN sanctions, had achieved its goal of being able to fit a miniaturised nuclear warhead on a rocket.
"Our nuclear scientists staged a nuclear explosion test on a newly developed nuclear warhead at the country's northern nuclear test site," a North Korean TV announcer said.
"Our... party sent a congratulatory message to our nuclear scientists... for conducting the successful nuclear warhead explosion test," she said as the South judged it was the North's biggest test yet at 10 kilotons.
The move drew swift condemnation from US President Barack Obama who warned of "serious consequences" and said he had called the leaders of South Korea and Japan to confer over the crisis.
President Park Geun-Hye spoke out against the "maniacal recklessness" of Kim, who since taking control after the death of his father in 2011 has carried out a series of purges and weapons tests designed to show strength and consolidate power.
"Kim Jong-Un's regime will only earn more sanctions and isolation... and such provocation will further accelerate its path to self-destruction," Park said, warning his obsession with creating a nuclear arsenal posed a grave challenge.
"We will step up pressure on the North by using all possible measures, including more, stronger sanctions on the North with the international community and at the UN Security Council," she said.
- Search for clues -News of the test emerged when seismic monitors detected a 5.3-magnitude "artificial earthquake" early Friday near the North's Punggye-ri nuclear site, where the last test took place in January.
"The 10-kiloton blast was nearly twice the fourth nuclear test and slightly less than the Hiroshima bombing, which was measured about 15 kilotons," said Kim Nam-Wook from the South's meteorological agency.
If Pyongyang can make a nuclear device small enough to fit on a warhead, and bolster the range and accuracy of its missiles, it might achieve its oft-stated aim of hitting US targets. But its claims to that in the past have been discounted.
Scientists will now attempt to analyse the blast to try to determine what kind of a breakthrough it represents, including whether it is a standard atomic bomb or a more powerful hydrogen, or thermonuclear, bomb.
Japan's public broadcaster NHK reported the country's defence ministry was preparing to dispatch aircraft to analyse air samples to see if any radiation could be detected.
"North Korea's carrying out of a nuclear test is absolutely unacceptable for Japan," said Prime Minister Shinzo Abe.
"North Korea's nuclear development is becoming a graver threat to Japan's safety and severely undermines the peace and safety of the region and the international community."
- 'Caught off guard' –
North Korea has been hit by five sets of United Nations sanctions since it first tested a nuclear device in 2006, but has insisted it will continue come what may, arguing it faces an existential threat from US aggression. 
Nuclear tests are usually heralded by chatter among analysts about preparations at Punggye-ri but there had been little discussion in recent weeks over any signs.
"The test caught many off guard, although Seoul officials have for months maintained that the North was maintaining a condition in which it was ready to stage a nuclear test any time," said  Kim Jin-Moo from the Korea Institute for Defense Analyses.
Pyongyang test-fired three missiles on Monday as world powers gathered for a G20 meeting in China's Hangzhou, with Kim Jong-Un hailing the tests as "perfect", and US President Barack Obama warning it would only up the pressure.
Kim Jin-Moo said the missile tests, including one launched from a submarine that far exceeded the range of previous such exercises, were a precursor to this "show of force" that had planned for months.
The test is another slap in the face to the North's chief ally China which has been under pressure to rein in its increasingly embarrassing behaviour.
China said Friday it "firmly opposes" the test but it has limited room to manoeuvre, given its priority is to avoid a collapse of the regime that would create a crisis on its border and shift the balance of power on the Korean peninsula towards the US.
The US-Korea Institute at Johns Hopkins University, which had warned Thursday of "new activity" at Punggye-ri, said the latest test made plain that the US and South Korean strategy on restraining North Korea "has clearly failed".
"No one should be surprised that North Korea continues to conduct nuclear tests to enhance the capabilities of its growing arsenal. Nor should they expect China to solve this problem for the United States," said the institute's Joel Wit.
North Korea claimed its January test was of a miniaturised hydrogen bomb, which can be far more powerful than other nuclear devices. 
But scientists say the estimated yield of around six kilotons was similar to the North's previous nuclear test in 2013, and far too low for a thermonuclear device. Initial indications from Monday's blast pointed in the same direction.
"What we can tell from the seismic waves so far is that this is likely not a thermonuclear test," said 38 North managing director Jenny Town.
"We are still in the preliminary stages but our analysts believe they tested a basic nuclear device not an advanced device," she said. "If we were looking at a thermonuclear test, we would see very different seismic waves."
© AFP