International condemnation has followed North Korea's claim it has successfully tested a hydrogen, or "H" bomb.
If confirmed, such a test would mark a major upgrade in North Korea's capabilities.
But there are questions about the claim.
Brianna Roberts reports.
It was the announcement from North Korea -- the Democratic People's Republic of Korea -- that it had successfully conducted a test of a miniaturised hydrogen nuclear device, or H-bomb.
And it set off alarm bells around the world.
The United Nations Security Council has held an emergency meeting, and UN secretary-general Ban Ki-moon has condemned the test, calling it "deeply troubling."
"This test once again violates numerous Security Council resolutions, despite the united call by the international community to cease such activities. It is also a grave contravention of the international norm against nuclear testing. This act is profoundly destabilising for regional security and seriously undermines international non-proliferation efforts. I condemn it, unequivocally. I demand the DPRK cease any further nuclear activities and meet its obligations for verifyable denuclearisation."
Suspicion that an underground test had been carried out was first raised after the United States Geological Survey detected unusual seismic activity in the north-east of the country.
Hours after a 5.1-magnitude tremor was detected near a nuclear testing site, the newsreader on North Korean state TV announced the testing of the country's first hydrogen bomb.
But some are skeptical about whether North Korea's claims are true.
A United States senior defense analyst at the RAND Corporation, Bruce Bennett, has questioned whether the size of the blast was large enough to be from an H-bomb.
"The bang they should've gotten would've 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 fusion part didn't work very well."
It could be days or weeks before independent tests are able to verify or dismiss North Korea's claim.
Both China and Japan have been trying to detect radiation.
Nevertheless, North Korea's claims -- and its clear nuclear ambitions -- are causing concern.
A spokeswoman for European Union foreign-policy chief Federica Mogherini, Maja Kocijancic, says such a test would threaten the stability of the region.
"If confirmed, this action would represent a grave violation of DPRK's international obligations not to produce or test nuclear weapons, as determined in several UN Security Council resolutions, and also a threat to the peace and security of the entire North-East Asia region and beyond."
Condemnation has been unanimous across the international community.
Even North Korea's main ally, neighbouring China, says it firmly opposes the test, urging North Korea to commit to denuclearisation.
Russia has warned the action could amount to what it calls "a severe violation of international law."
And yet another neighbour, Japan, has called it a major threat to its national security.
The United States and South Korea have expressed agreement that what they call North Korea's provocations should have consequences.
South Korean president Park Geun-Hye says her government is prepared to act.
(Translated)"So now, our government has to take decisive measures against any additional provocations by North Korea and work with the international community to make sure the isolated country pays the price for its latest nuclear test."
North Korea has previously successfully tested fission, or atom-splitting, bombs, known as atom bombs.
Hydrogen bombs use fusion, or atomic nuclei coming together.
They are much more powerful -- a thousand times or more -- and have the potential to cause much more damage.
Hydrogen bombs can be made small enough to fit on the head of an intercontinental missile, but they require more technology as far as control and accuracy.
They are what the five primary nuclear powers -- the United States, Russia, China, France and Britain -- have, but the H-bomb has never been used in warfare.
The United States is the only country to have ever used an atom bomb in war, dropping two on Japan at the end of the Second World War in 1945.
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