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.
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.
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