Beijing urged restraint on Thursday after Pyongyang said it is planning a third "higher level" nuclear test and more rocket launches aimed at the US.

Washington said the nuclear test, if staged, would be "a mistake and a missed opportunity" for Pyongyang.

Experts said the planned nuclear test is likely to use highly enriched uranium instead of the plutonium used in Pyongyang's previous tests. The test date will probably be Feb 16, the birthday of Kim Jong-il, or April 15, the birthday of Kim Il-sung.