Following are the key dates in North Korea's nuclear programme build-up:
1945: The end of World War II finds Korea divided at the 38th parallel between North, backed by the Soviet Union, and South, supported by the United States.
1950-53: After Soviet and US troops leave the Korean peninsula, war breaks out between North and South. The United States intervenes on behalf of the South, and newly communist China on the side of the North.
The Korean War leaves an estimated four million people dead, and results in North Korea becoming one of the most tightly controlled societies in the world.
The US retains a huge military presence - believed to include nuclear weapons - on the southern side of the border.
1968: The USS Pueblo, an intelligence-gathering vessel, is seized by North Korean gunboats, sparking a tense standoff with the US. The crew of 83 Americans are detained for 11 months before being released.
1969: North Korea shoots down an American reconnaissance plane.
1988: The US imposes sanctions on North Korea after putting the country on its list of nations supporting terrorism.
1989: US satellite pictures reveal a nuclear reprocessing plant at North Korea's Yongbyon complex. Washington accuses North Korea of actively pursuing nuclear weapons. Pyongyang denies the charge.
1994: North Korea and the US sign a nuclear safeguard accord after Pyongyang vows to freeze and dismantle its nuclear weapons program in return for the construction of safe nuclear reactors.
1999: North Korean leader Kim Jong-II declares a moratorium on missile tests. The US eases its sanctions.
2000: US Secretary of State Madeleine Albright meets Kim Jong-II during a historic two-day visit to Pyongyang. High level visits between US and North Korean officials appear to indicate a warming of ties. At the same time, relations between North and South Korea begin to warm noticeably.
2002: Ties worsen sharply as US President George W Bush names North Korea as part of an "axis of evil", alongside states such as Iran and Iraq.
2003: North Korea withdraws from the Nuclear Non-Proliferation Treaty and threatens to carry out a nuclear test. Six-way nuclear talks start in Beijing, after the United States refuses to hold direct negotiations with North Korea.
2005: For the first time, North Korea explicitly states that it possesses nuclear weapons. Indirect talks nevertheless continue, still with no conclusion.
2006: North Korea carries out test-firings of long-range missiles, leading the United Nations Security Council to declare sanctions against it.
October 3, 2006: North Korea again threatens to carry out a nuclear weapons test, saying it is under "extreme threat of a nuclear war and sanctions" from the United States.
October 6, 2006: UN Security Council unanimously adopts a call for North Korea to abandon plans to test a nuclear bomb.
October 9, 2006: North Korea announces it has carried out its first test of an atomic bomb.