The United Nations Security Council says it will "begin to work immediately" to take action  about against by during for in of on out to    North Korea. The UN believes a nuclear test that North Korea carried  about against by during for in of on out to    on Wednesday could be a hydrogen bomb. This was the fourth time since 2006 that North Korea has tested a nuclear bomb, but it could be the first time it has tested a hydrogen bomb. A hydrogen bomb is a lot more powerful than the atomic bombs dropped  about against by during for in of on out to    Japan  about against by during for in of on out to    World War II. The UN said  about against by during for in of on out to    North Korea's test that: "A clear threat  about against by during for in of on out to    international peace and security continues to exist." However, the UN has not said what action it would take  about against by during for in of on out to    North Korea, nor when any measures would be put  about against by during for in of on out to    place. Not all countries agree  about against by during for in of on out to    what to do  about against by during for in of on out to    North Korea's latest nuclear test. The USA said it does not think the test was a hydrogen bomb because the explosion was too small. A South Korean politician agreed. However, both the USA and South Korea said the test was a dangerous move  about against by during for in of on out to    North Korea. Japan wants the UN to act quickly to take action. Japan's ambassador  about against by during for in of on out to    the UN said: "The authority and credibility  about against by during for in of on out to    the [UN] will be put  about against by during for in of on out to    question if it does not take these measures." China had strong words  about against by during for in of on out to    North Korea. Its government said: "We strongly encourage North Korea to honour its de-nuclearisation promises and to stop taking any action that will make this situation worse."