North Korea said it successfully conducted its second nuclear test on Monday, a move seen as certain to further isolate the impoverished state.
North Korea is already under UN Security Council sanctions for its first nuclear test in 2006, which officials and analysts say was only partially successful.
Here is a timeline of the North's nuclear activities and international attempts to contain them.
* 1979: North Korea starts to build a 5-megawatt nuclear reactor at Yongbyon, aided by years of Soviet nuclear help.
* Dec 12, 1985: North Korea declares existence of the Yongbyon facility to the International Atomic Energy Agency (IAEA), joins the nuclear Non-Proliferation Treaty (NPT). Treaty requires IAEA inspectors have regular access for monitoring.
* 1986: Yongbyon begins operation.
* March 1993: North Korea rejects IAEA inspectors' request to see two undeclared sites and threatens to withdraw from the
* June 1994: North Korea quits the IAEA, 20 years after it first became a member state.
* October 21: U.S. President Bill Clinton's administration signs an Agreed Framework with North Korea to freeze its nuclear program in exchange for aid.
* Dec 12, 2002: North Korea says it plans to restart Yongbyon reactor. By the end of the month it has disabled the IAEA surveillance devices there and expels the agency's inspectors.
-- The move comes two months after U.S. State Department envoy James Kelly confronts Pyongyang with evidence Washington says points to a covert uranium enrichment program.
* January 2003: North Korea quits the NPT.
* Sept 2005: North reaches deal with China, Japan, Russia, South Korea and United States on "abandoning all nuclear weapons and existing nuclear programs and returning, at an early date, to the NPT and to IAEA safeguards" at talks in Beijing.
* Oct 9, 2006: North carries out first nuclear test.
* February 13, 2007: North Korea agrees to start shutting its reactor and allow U.N. nuclear inspectors back into country in exchange for aid, the first step in a new disarmament deal.
* August 26, 2008: Pyongyang says it will reverse disablement of Yongbyon facilities, angered by a Washington decision not to take it off its terrorism blacklist.
* Oct 11: U.S. says it will take North Korea off state sponsors of terrorism list, following verbal agreement on dismantlement. North says next day to resume disablement. IAEA soon granted access to key Yongbyon plants.
* April 5, 2009: North Korea launches a multistage rocket. A week later, on the 13th, the U.N. security council adopts a declaration condemning North Korea for the launch.
* April 14: North Korea's foreign ministry says the country will quit six-party nuclear talks, and re-start Yongbyon. It orders U.N. inspectors to leave.
* April 25: North Korea's foreign ministry says reprocessing of spent fuel rods from the pilot atomic power plant has begun.
* April 29: North Korea threatens to conduct nuclear test and also test an intercontinental ballistic missile unless the U.N. Security Council apologizes for imposing sanctions against it.
* May 25: North Korea says it has successfully conducted a nuclear test, raising the explosive power and level of control of its nuclear device to a new level.