Here are some facts about underground nuclear tests and the international treaties related to them:
North Korea's state news agency announced on Monday that the country had conducted its first nuclear test, less than a week after Pyongyang threatened to do so.
h Korea shocks world by saying it will quit Nuclear Nonproliferation Treaty, later suspends its withdrawal.
The second round of the six-party talks on nuclear issue on the Korean Peninsula will open in Beijing on Feb. 25. The following is a chronology of key events in the nuclear issue on the Korean Peninsula since 1991.