North Korea nuclear: State claims first hydrogen bomb test
North Korea says it has successfully carried out an underground hydrogen bomb test which, if confirmed, would be its fourth nuclear test since 2006.
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The claim came after a 5.1 magnitude tremor was detected close to its nuclear test site at Punggye-ri, which monitors said was likely not natural.
This is North Korea's first claim to have tested a hydrogen bomb, which is more powerful than an atomic bomb.
But international experts say there is no confirmation as yet. Leaders condemn reports of hydrogen bomb test.
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.
North Korea has a determination to defy both world opinion and heavy sanctions to reach their nuclear goal. Crucially, its main ally, China, has proved either unwilling or unable to help.
Suspicion an underground test had been carried out was first raised after the US Geological Survey said the epicentre of the quake - detected at 10:00 Pyongyang time (01:30 GMT) - was in the north-east of the country, about 50km (30 miles) from Kilju city, near Punggye-ri.
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Then 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."
Last month, North Korean leader Kim Jong-un said Pyongyang had developed a hydrogen bomb, although many experts were sceptical.
It could be days or weeks before independent tests are able to verify or dismiss the claim.
Hours before the seismic activity, South Korean media reported that Pyongyang had test-fired a ballistic missile from a submarine in late December. It is unclear whether the test was successful or not.
North Korea said in May it had conducted a similar successful test.
A hydrogen bomb would be is a step up in destructive power from previous tests. It gives more explosive power for a lighter weight.
After the last test, in 2013, there was widespread condemnation. The UN Security Council held an emergency meeting at which its members, including China, "strongly condemned" it. Similar outrage is expected this time. Prime Minister Shinzo Abe of Japan has said this was a "serious threat" to the safety of his nation.
On top of any fourth nuclear test, North Korea also appears to have tested a submarine-launched missile. The ability to launch missiles from submarines would change the whole calculation of military response because warning times of an attack on, for example, the West Coast of the United States would be much shorter.
Before the test, North Korean state media said the country "deserved to hold nuclear weapons... to counter nuclear threats by the US".
Experts had believed North Korea was still some years from being able to hit a target with a nuclear bomb delivered by a missile. But it is crystal clear that it is absolutely determined to be able to do so. It is also clear that it is improving its abilities rapidly.
How advanced is North Korea's nuclear programme?
Strong reaction started pouring in after the North Korean announcement of the hydrogen bomb test, with South Korea saying it was a serious challenge to global peace and a violation of UN Security Council resolutions.
China, North Korea's main ally, said it "firmly opposes" the test saying it was carried out "irrespective of the international community's opposition".
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The Japanese Prime Minister Shinzo Abe said it was a threat to Japan's safety and could not be tolerated. The US called on North Korea to abide by its international commitments and obligations saying it would respond to provocations.