The US has joined condemnation of North Korea after it tested what it says is a more advanced nuclear weapon.
Pyongyang said it had successfully trialled a hydrogen bomb that could be loaded on to a long-range missile.
President Donald Trump tweeted that North Korea’s “words and actions” were “very hostile and dangerous”.
North Korea has defied UN sanctions and international pressure to develop nuclear weapons and test missiles that could potentially reach the US.
South Korea, China and Russia have all voiced strong criticism of the North’s sixth nuclear test.
What has happened?
The first suggestion that this was to be a far from normal Sunday in the region came when seismologists’ equipment started picking up readings of an earth tremor in the area where North Korea has conducted nuclear tests before.
The US Geological Survey put the tremor at 6.3 magnitude.
Japanese Foreign Minister Taro Kono said there was no doubt this was North Korea’s sixth nuclear test.
Then North Korean state media confirmed this was no earthquake.
It claimed the country had conducted its sixth and most powerful nuclear test, detonating a hydrogen bomb that could be loaded onto a long-range missile.
North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb.
Analysts say North Korea’s claims should be treated with caution, but that its nuclear capability is clearly advancing.
What has the reaction been?
Denouncing the test as “hostile” and “dangerous”, President Trump described the North as a “rogue nation” which had become a “great threat and embarrassment” to China – Pyongyang’s main ally.
He also said South Korea’s “talk of appeasement” was not working and that the secretive communist state “only understands one thing”.
The White House later said the US president would hold a national security meeting on Sunday.
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South Korean President Moon Jae-in called for the “strongest possible” response, including new UN Security Council sanctions to “completely isolate” the country.
“I can’t help but be disappointed and outraged,” he said, adding that North Korea’s weapons programme was “threatening world peace” and would only “isolate them further”.
China, meanwhile, also expressed “strong condemnation” and said the state “had ignored the international community’s widespread opposition”.
Russia urged all sides involved to hold talks, saying this was the only way to resolve the Korean peninsula’s problems.
UK Foreign Minister Boris Johnson denounced the “reckless” test and warned that being able to fit a warhead to a missile would present a “new order of threat” from Kim Jong-un’s regime.
What does the test tell us?
South Korean officials said the latest test took place in Kilju County, where the North’s Punggye-ri nuclear test site is situated. The “artificial quake” was 9.8 times more powerful than the tremor from the North’s fifth test in September 2016, the state weather agency said.
Although experts urged caution, this does appears to be the biggest and most successful nuclear test by North Korea to date – and the messaging is clear. North Korea wants to demonstrate it knows what makes a credible nuclear warhead.
Nuclear weapons expert Catherine Dill told the BBC it was not yet clear exactly what nuclear weapon design was tested.
“But based on the seismic signature, the yield of this test definitely is an order of magnitude higher than the yields of the previous tests.”
Current information did not definitively indicate that a thermonuclear weapon had been tested “but it appears to be a likely possibility at this point”, she said
Hydrogen bombs are many times more powerful than an atomic bomb. They use fusion – the merging of atoms – to unleash huge amounts of energy, whereas atomic bombs use nuclear fission, or the splitting of atoms.
What can be done?
By Jonathan Marcus, BBC defence and diplomatic correspondent
North Korea’s sixth nuclear test – probably its largest so far – sends out one clear political signal.
Despite the bluster and threats from the Trump administration in Washington and near-universal condemnation from around the world, Pyongyang is not going to halt or constrain its nuclear activities.
Worryingly, it also suggests that this is a programme that is progressing on all fronts at a faster rate than many had expected. So far all efforts to pressure North Korea – sanctions, isolation and military threats – have all failed to move Pyongyang.
Could more be done? Certainly, but the harshest economic pressure would potentially cripple the regime and push it towards catastrophe – something China is unwilling to countenance.
Containment and deterrence will now come to the fore as the world adjusts its policy from seeking to roll-back Pyongyang’s weapons programme to living with a nuclear-armed North Korea.
Previous nuclear tests
Will China clamp down?
By Robin Brant, BBC News, Shanghai
North Korea’s sixth nuclear weapons test is an utter rejection of all that its only ally has called for.
Beijing’s response was predictable – condemnation, urging an end to provocation and dialogue. But it also spoke of urging North Korea to “face up to the firm will” of the international community to see denuclearisation on the Korean peninsula.
There is no sign, though, that China is willing yet to see that “firm will” go beyond UN sanctions, which recently clamped down on seafood and iron ore exports, in addition to the coal and minerals that are already banned from crossing the border.
It is noteworthy also that this test took place just as the Chinese president was about to welcome a handful of world leaders to the two-day showpiece Brics summit on China’s east coast.
Even the state-controlled media will find it hard to ignore the fact that their man has been upstaged – embarrassed too – by its almost universally ostracised ally and neighbour.