The United Nations Security Council says it will "begin to work immediately" to take action against North Korea. The UN believes a nuclear test that North Korea carried out 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 on Japan during World War II. The UN said of North Korea's test that: "A clear threat to international peace and security continues to exist." However, the UN has not said what action it would take against North Korea, nor when any measures would be put in place.