North Korea announced a successful test of Hydrogen Bomb

North Korean leader Kim Jong-un

After a nuclear 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.

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 skeptical.

What is a hydrogen bomb?

A weapon energized 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