The United Nations says it will begin work immediately to take action against North Korea. The UN believes a nuclear test that North Korea carried out could be a hydrogen bomb. This was the fourth time that North Korea tested a nuclear bomb. It could be the first time it has tested a hydrogen bomb, which is a lot more powerful than the atomic bombs dropped during World War II. The UN said: "A clear threat to international peace and security continues to exist." The UN has not said what action it would take against North Korea, nor when they would be put in place.

Not all countries agree on what to do about North Korea. The USA said it does not think the test was a hydrogen bomb because the explosion was too small. Both the USA and South Korea said the test was a dangerous move by North Korea. Japan wants the UN to act quickly. It said: "The authority and credibility of the [UN] will be put in question if it does not take these measures." China had strong words for North Korea. It said: "We strongly encourage North Korea to honour its de-nuclearisation promises and to stop taking any action that will make this situation worse."