N Korea still short of nuclear goal

North Korea says it has successfully carried out a nuclear test, but that does not necessarily mean it is now capable of firing a nuclear bomb at a target.

The reported underground test in the north-east of the country is just one of a series of procedures that need to be carried out to stage a fully-fledged nuclear attack.

One of the most complicated parts of the process involves miniaturising a nuclear device so it can fit on a missile.

Many defence analysts believe North Korea does not yet have the capability to do that.

The process involves fitting to a missile what is called a "bus" on which the warheads sit, covered by a cone, said Dr Lee Willett, a defence analyst at the Royal United Services Institute.

It is a complex procedure that fundamentally changes the missile, giving it a different weight and consequently a new trajectory.

But it is something North Korea will be working on, Dr Willet believes. "I don't think they do anything in half measures," he said.

Missile tests

Prototype nuclear weapons in the past have been physically very large and North Korea's is likely to be the same, James Acton, of the Verification Research, Training and Information Centre (Vertic) in London, says.

"If North Korea has just tested a first generation device then it is currently likely to be too big to fit on a missile," he said.

A large bomb could be transferred by truck, ship or, with modifications, dropped from a plane, he said.

But it is thought North Korea's preferred way would be to fit a warhead to a missile, which reach their target faster and are harder to shoot down.

In July this year, North Korea test-fired seven missiles, including a failed test on a long-range Taepodong-2 thought capable of reaching the US.

A report by the Institute for Science and International Security (ISIS), a month earlier, said there was little evidence to suggest North Korea was capable of making a nuclear warhead light enough for the Taepodong-2.

But it did say North Korea could have the capability to build a crude nuclear warhead for its medium-range Nodong missile.

The Nodong missile could hit most of Japan and has a high margin of error. It is feared that if fired on a military target, its inaccuracy could led to high levels of civilian casualties.

Meanwhile, neighbouring South Korea is the most vulnerable. Any number of Pyongyang's missiles could be fired over the border, Mark Fitzpatrick of the International Institute of Strategic Studies says.