Although construction work began on the reactors, the project has since been cancelled after North Korea resumed reprocessing work at Yongbyon and restarted its reactor.

Building the bomb

Basic nuclear fission devices can be made using either uranium or plutonium.

Uranium is a naturally occurring element which is a key ingredient in nuclear power generation, and, in a highly enriched form (uranium-235) in nuclear weapons. Enrichment usually takes place in a series – or "cascade" – of high precision centrifuges.

 Plutonium is a rare and highly radioactive element. Although extremely tiny amounts occur naturally, the isotope plutonium-239 used in nuclear weapons can only be produced by reprocessing "spent" uranium fuel rods used in a nuclear reactor.

US intelligence officials believe the Yongbyon facility has produced about 50kg of plutonium, which they believe would be enough for about eight nuclear weapons.

The existence of these weapons has never been confirmed, but recent North Korean statements have referred to the country being in possession of a "stockpile" of nuclear weapons.

North Korea had not publicly admitted to possessing nuclear bombs until February 2005, when a statement released in North Korean media warned the country planned to "bolster its nuclear weapons arsenal".

In October 2006, however, the North stunned the world when it conducted its first nuclear weapon test with a plutonium-based bomb.

The underground test was considered small by the standards of established nuclear powers, leading to widespread debate among experts as to how successful the test actually was.

Four years earlier, US officials confronted North Korea, accusing it of running a clandestine programme to enrich uranium for nuclear weapons, contrary to previous commitments and in parallel to its plutonium enrichment efforts.

North Korea, which until recently had not publicly admitted to having such a programme, has ample supplies of natural uranium and could, in theory, enrich it to weapons grade away from the gaze of spy satellites.

How much uranium it has enriched and how far away it is from producing a uranium-based weapon are not clear.

Warhead doubts

US officials say they believe North Korea is developing long-range missiles [EPA]
Despite having now conducted two test nuclear detonations, experts say it remains doubtful that North Korea has mastered the technology to make a nuclear weapon in deliverable form – i.e. small enough to mount on a warhead or drop from an aircraft.

The North does though have an active missile development programme and is working on a long range missile known as the Taepodong-2.

In April it tested a three-stage version of the rocket, which it says was part of a peaceful space programme aimed at placing a communications satellite in orbit.

US intelligence officials however say no satellite has been detected and the launch was a cover for a test of the North's missile technology.

The missile may eventually put the US within range of a North Korean strike.

Another option for delivering a nuclear weapon would be by aircraft, but according to South Korean defence estimates, the North Korean air force, while large in numbers, is outdated and in relatively poor shape.

It is thought to have around 780 ageing Soviet-era fighters and 80 bombers.