SEOUL North Korea's Foreign Ministry called on Monday for direct talks with its long-time foe, the United States, and said it was ready to return to stalled six-way nuclear disarmament negotiations.
The following is a look at destitute North Korea's decades-long pursuit of nuclear arms:
The Yongbyon complex is at the heart of the North's plutonium weapons programme. It consists of a five-megawatt reactor, whose construction began in 1980, a fuel fabrication facility and a plutonium reprocessing plant, where weapons-grade material is extracted from spent fuel rods.
Under an earlier agreement, North Korea began to close down the facility, but this year announced that in the face of U.S. hostility it would restore parts of the plant.
The site, about 100 km (60 miles) north of Pyongyang, also contains a 50-megawatt reactor whose construction was suspended under a 1994 nuclear deal with the United States. That reactor is nowhere near completion.
When fully operational, Yongbyon can produce enough plutonium for one nuclear bomb a year, experts say.
U.S. officials said prior to the North's May 25 second nuclear test it had produced about 50 kg (110 lb) of plutonium, which proliferation experts say would be enough for six to eight nuclear weapons. It could eventually extract enough material from spent fuel rods is says it is reprocessing at Yongbyon to make one more bomb.
North Korea's first nuclear test in October 2006 produced a relatively low yield in its explosive force, indicating problems with the bomb design or plutonium at its core, experts say.
The May 25 test was stronger, but experts believe it may only be about one-fifth as powerful as the plutonium bomb the U.S. dropped on the Japanese city of Nagasaki in 1945 at the end of World War Two.
Even though it has exploded nuclear devices, North Korea has not shown that it has a working nuclear bomb.
Experts said they do not believe the North has the ability to miniaturise an atomic weapon to place on a missile, but the secretive state has been trying to develop such a warhead.
Even if it did, North Korea does not appear to have the technology to guide the missile to a target.
Its ageing fleet of Soviet-era bombers would have difficulty evading the technologically advanced air forces of regional powers the United States, South Korea and Japan to deliver a nuclear bomb outside the country. It has been testing ballistic missiles capable of reaching U.S. territory, but these have so far failed.
About two months ago, the North announced it was close to completing experimental enrichment of uranium, something the United States long suspected it was doing and giving it another path towards an atomic bomb.
The advantage for the North is that, unlike plutonium extraction from spent nuclear fuel, enrichment can much more easily be done away from the prying eyes of U.S. spy satellites. Also, the North can fuel it with its ample supplies of natural uranium.
(Reporting by Jon Herskovitz and Jonathan Thatcher)