Here are some facts about North Korea's nuclear programme:
North Korea's nuclear programme is centred at Yongbyon, about 100km north of Pyongyang, the capital. The complex consists of a five-megawatt reactor and a plutonium reprocessing plant, where weapons-grade material would be extracted from spent fuel rods.
Extracting fissile material
Experts and intelligence reports indicated that the Democratic People's Republic of Korea had extracted enough fissile material from Yongbyon to produce one or two nuclear weapons by the early 1990s.
In October 1994, the US and North Korea struck a deal to freeze the Yongbyon complex in exchange for more proliferation-resistant reactors to be built by an international consortium. That project has been cancelled.
In October 2003, Pyongyang said it had enhanced its nuclear deterrent by reprocessing 8,000 spent fuel rods from Yongbyon. US intelligence experts said the North could extract enough fissile material from those rods for another four to six weapons.
In February 2005, North Korea declared for the first time that it had nuclear weapons.
In May 2005, North Korea said it had extracted more fuel rods from Yongbyon. Proliferation experts said this could eventually provide enough material for another two or three atomic bombs.
A conservative estimate would be that North Korea has enough fissile material for at least six to eight nuclear weapons, proliferation experts have said, with some saying it could have enough for more than a dozen.
Delivering a weapon
It is impossible to say whether North Korea has built a workable nuclear weapon. Experts have said the secretive state has conducted many tests on nuclear bomb-related technologies.
North Korea has an extensive missile programme, but no one is sure if the country can make a nuclear weapon small enough to mount on a warhead.
North Korea test-fired seven missiles on July 5, including its long-range Taepodong-2 with a range some experts said could one day reach US territory.
On October 9, North Korea's official news agency reported that a successful underground test had been carried out.
"Our science research section has safely and successfully conducted an underground nuclear test on October 9," it said.