International scepticism and condemnation have greeted North Korea's claim to have successfully carried out an underground hydrogen bomb test.
If confirmed, it would be North Korea's fourth nuclear test since 2006 and mark a major upgrade in its capabilities.
But nuclear experts have questioned whether the size of the blast was large enough to have been from an H-bomb.
UN chief Ban Ki-moon condemned the test "unequivocally", calling it "profoundly destabilising for regional security".
The UN Security Council held an emergency meeting on Wednesday and vowed to work immediately on a new resolution.
South Korea called the test a "grave provocation" but said it was difficult to believe it was an H-bomb.
Hydrogen bombs are more powerful and technologically advanced than atomic weapons, using fusion - the merging of atoms - to unleash massive amounts of energy.
Atomic bombs, like the kind that devastated two Japanese cities in World War Two, use fission, or the splitting of atoms.
Bruce Bennett, an analyst with the Rand Corporation, was among those casting doubts on Pyongyang's test: "The bang they should have gotten would have been 10 times greater than what they're claiming.
"So Kim Jong-un is either lying, saying they did a hydrogen test when they didn't, they just used a little bit more efficient fission weapon - or the hydrogen part of the test really didn't work very well or the fission part didn't work very well."
The data "doesn't support suggestions that the bomb was a hydrogen bomb", Chinese military expert Du Wenlong told state broadcaster CCTV.
A South Korean politician, Lee Cheol-woo, said he was briefed by the country's intelligence agency that the blast "probably falls short" of a hydrogen detonation.
But former British ambassador in Pyongyang John Everard warned "an explosion of that size is quite enough to wipe out a city and I think that, of course, is deeply worrying".
South Korean President Park Geun-hye, said further analysis was needed to determine the nature of the test, while calling it "a strong challenge to international peace and stability". In other reaction:
- China, North Korea's main ally, said it "firmly opposes" the test
- Japan called it a "major threat" to its national security
- The US and South Korea agreed that "North Korea's provocations should have consequences"
- Russia warned the action could amount to "a severe violation of international law", calling for the resumption of talks
- The EU urged North Korea "cease this illegal and dangerous behaviour''
- Nato said North Korea should abandon nuclear weapons
Heading into the UN meeting, Mr Ban said the test was "deeply troubling and "a grave contravention of international norms".
The UK ambassador to the UN, Matthew Rycroft, said: "We will be working with others on a resolution on further sanctions."
North Korea's 'spectacular' rhetoric, by the BBC's Steve Evans
The rhetoric from the North Korean media was spectacular, announcing the country had carried out a "world startling event" - the underground test of a hydrogen bomb.
"People of the DPRK are making a giant stride, performing eye-catching miracles and exploits day by day," state media said.
That North Korea is still living with its predictable 1950s post-Korean War world view, where the US is the prime aggressor, was made clear too.
"The US is a gang of cruel robbers which has worked hard to bring even a nuclear disaster to the DPRK."
But despite the rhetoric, outside experts are sceptical about how much of a giant stride had been made.
What is not in doubt is the determination of Pyongyang to go down the nuclear path despite widespread condemnation the last time it tested a device.
Suspicions first emerged when an earthquake was registered near the Punggye-ri nuclear site in North Korea at 10:00 Pyongyang time (01:30 GMT), with the tremors rattling Chinese border cities.
Hours later, in a surprise announcement, a newsreader on North Korean state TV said: "The republic's first hydrogen bomb test has been successfully performed at 10:00 am on January 6, 2016."
A note signed by North Korea leader Kim Jong-un authorising the test said 2016 should begin with the "stirring explosive sound" of a hydrogen bomb.
It could be days or weeks before independent tests are able to verify or dismiss the recent claim.
Both China and Japan are reported to have been trying to detect radiation.
North Korea carried out the first of its three previous nuclear tests in 2006, making it one of the few nuclear-armed nations on Earth.
Can North Korea now launch a nuclear missile?
Despite North Korea's claims, experts are sceptical that North Korea can make a nuclear weapon small enough to fit on a missile.
What do we know about the latest test?
Observers agree a nuclear explosion of some kind took place and it seems to have been a bit bigger than the last test in 2013, but not nearly big enough to be a full thermonuclear explosion - an "H-bomb" - as Pyongyang claims.
Why can't the world stop North Korea?
North Korea has a determination to defy both world opinion and heavy sanctions to reach its nuclear goal. Crucially, its main ally, China, has proved either unwilling or unable to help.