The US, South Korea and Japan have said they will be united in their response to North Korea’s claim to have successfully tested a hydrogen bomb, the BBC reports.

North Korea said it carried out the test on Wednesday – if confirmed it would be its fourth nuclear test, and its first of the more powerful H-bomb.

The UN Security Council has also agreed to start drawing up new measures against North Korea.

But scepticism remains over whether the North really did conduct such a test.

Experts have said the seismic activity generated by the blast was not large enough for it to have been a full thermonuclear explosion.