The United Nations (UN) Security Council has begun working on a resolution proposing further measures against North Korea.

The US is calling for more sanctions after the country claimed it had successfully detonated a hydrogen bomb. America says it doubts the claims are true, though.

And South Korea's President Park Oon Eye has reacted angrily to its neighbour's actions:

"We, and the international community, have repeatedly warned Pyongyang that if it goes through with the nuclear test we will not ignore it and we will make the country pay for its action."