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.

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