North Korea added its 5-megawatt plutonium reactor to its nuclear complex at Nyongbyon in 1986, and Pyongyang is believed to have exploded plutonium devices in its first two nuclear tests, in 2006 and 2009.
There had long been claims by the U.S. and others that Pyongyang was also pursuing a secret uranium program. In 2010, the North unveiled to visiting Americans a uranium enrichment program at Nyongbyon.
Analysts say they don't believe North Korea currently has mastered the miniaturization technology needed to build a warhead that can be mounted on a missile, and the extent of its uranium enrichment efforts is also unclear.
Scientist and nuclear expert Siegfried Hecker, one of the Americans on the 2010 visit to Nyongbyon, has estimated that Pyongyang has 24 to 42 kilograms of plutonium — enough for perhaps four to eight rudimentary bombs similar to the plutonium weapon used on Nagasaki in World War II.
It's not known whether the North's latest atomic test, in February, used highly enriched uranium or plutonium stockpiles. South Korea and other countries have so far failed to detect radioactive elements that may have leaked from the test and which could determine what kind of device was used.
Associated Press writers Sam Kim and Jean H. Lee in Seoul and AP researcher Zhao Liang in Beijing contributed to this report.