Nuclear North Korea
North Korea has been under U.N. sanctions since its first nuclear test in 2006, but has defied pressure to abandon its missile and nuclear programs. The North has also been accused of using the nuclear programme to attempt to wring concessions and aid from the United States.
Facilities
Underground testing
The North's first test in October 2006 produced a relatively low yield. A second test, in 2009, was stronger, but experts believe it may have only been about one-fifth to one-fourth as powerful as the plutonium bomb the United States dropped on the Japanese city of Nagasaki in 1945. The U.N. Security Council expanded sanctions after a third nuclear test in February 2013.
Two routes to a nuclear bomb
After a nuclear test, monitoring sites will attempt to assess whether North Korea used highly enriched uranium which would give it a second path to a nuclear weapon.
Extracting plutonium
Pyongyang claims to be able to reprocess spent fuel rods at its Yongbyon nuclear plant and turn it into arms-grade plutonium, giving the mercurial state more material to produce atomic arms.
Enriching uranium
North Korea has claimed to be enriching uranium, giving it another path for making atomic weapons. Uranium enrichment can be conducted away from the prying eyes of satellites and the North can fuel it with its ample supplies of natural uranium.
The next stage
Nuclear testing is part of a process that moves Pyongyang closer to miniaturising nuclear warheads to mount on intercontinental ballistic missiles. North Korea has well over 1,000 missiles of various ranges.
Sources: Reuters; National Nuclear Security Administration; Global Security; Nuclear Threat Initiative; Federation of American Scientists; Center for Nonproliferation Studies; South Korean defence ministry; International Atomic Energy Acency; World Nuclear Association