The North Korean authorities say they have successfully tested a hydrogen bomb amid reports of a tremor near the main nuclear test site.
State media announced the test after monitors detected a 5.1 magnitude quake close to the Punggye-ri site.
The North is thought to have conducted three previous underground nuclear tests there since 2006.
A hydrogen bomb uses fusion to create a blast far more powerful than that of a more basic atomic bomb.
This test was apparently a hydrogen bomb, a step up in destructive
power from the plutonium used in previous tests. It gives more
explosive power for a lighter weight.
After the test in 2013,
there was widespread condemnation. The UN Security Council held an
emergency meeting at which its members, including China, "strongly
condemned" the test. Similar outrage is expected this time. Prime
Minister Abe of Japan has said this fourth test 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
believed before the fourth test that 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.
If confirmed, it would mean
Pyongyang is intent on pursuing its nuclear programme with little regard
for the major political and diplomatic costs that will inevitably
accompany this unwelcome development, says Dr John Nilsson-Wright of
Asia Programme at Chatham House.
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 international experts were
sceptical.
What is a hydrogen bomb?
A weapon energised by the nuclear fusion of hydrogen isotopes in a chain reaction, developed in 1958 by the United States
Also
known as a thermo-nuclear bomb, it is seen as a "cleaner" bomb than an
atomic one as it has less radioactive fallout - but also much more
powerful
Unlike
an atomic bomb, powered by nuclear fission, a hydrogen bomb is powered
by the fusion of lighter elements into heavier elements
Such bombs can be as small as a few feet long and can fit in warheads of ballistic missiles
Suspicion
of a test 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, some 50km (30 miles) from Kilju
city, near Punggye-ri.
The BBC's Kevin Kim in Seoul says analysts
will now focus on trying to detect if any gases have leaked from the
subterranean explosion to conclude what type of nuclear material may
have been used, if it indeed it was a test of a hydrogen bomb. North Korea's nuclear tests 9 October 2006
The
first test detonated a device based on plutonium, rather than enriched
uranium. The test was conducted at Punggye-ri, also known as
P'unggye-yok. US intelligence officials said that it had not been a
powerful one, measuring less than one kiloton - that is less then a
tenth of the size of the bomb dropped on Hiroshima. 25 May 2009
A
second underground nuclear test was carried out which was said to be
more powerful than the first. Russia's defence ministry estimated a
blast of up to 20 kilotons, a similar size to the American bombs that
completely destroyed Hiroshima and Nagasaki in 1945.
Although the
North gave no details of the test location, South Korean officials said a
seismic tremor was detected in its neighbour's north-east around the
town of Kilju - close to Punggye-ri. 12 February 2013
Unusual
seismic activity was detected around Punggye-ri. This was followed by
confirmation from the state news agency that North Korea had
successfully tested a device. The announcement referenced a
"miniaturised" nuclear device which raised fears that Pyongyang's
ultimate aim is to produce a device small enough to fit on a long-range
missile.
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