Korea nuclear stand-off

UN nuclear inspectors have travelled to North Korea for the first time since 2002, to discuss "shutting and sealing" the country's main reactor, Yongbyon.

The move is part of a deal agreed on 13 February, in which Pyongyang pledged to close Yongbyon in exchange for energy aid and other benefits.

Progress on the deal had been stalled for months over a banking row, but now the problem has finally been resolved.

BBC News looks at the background to the long-running nuclear dispute.

What was agreed in the 13 February deal?

North Korea said it would "shut down and seal" its main nuclear reactor at Yongbyon within 60 days. In return it would receive 50,000 tonnes of heavy fuel oil, to be supplied by the five other countries involved in the nuclear negotiations - the US, China, South Korea, Japan and Russia.

North Korea would then declare and disable all its existing nuclear facilities, in return for which it would receive a further 950,000 tonnes of heavy fuel oil.

The North also agreed to invite the IAEA to return to the country to monitor the agreement - something which has now taken place.

And it also said it would begin talks to normalise its diplomatic relations with the US and Japan.

These initial steps were meant to kick-start an earlier 2005 agreement, known as the Joint Statement, under which North Korea committed to abandoning all its nuclear weapons and nuclear programmes and returning to the treaty on the non-proliferation of nuclear weapons (NPT).

Why was there a hold-up?

North Korea refused to proceed with the February deal until it had access to $25m (£12.5m) which had been frozen in a Macau bank.

The funds were frozen after the US alleged they were linked to money-laundering and counterfeiting.

Although the US lifted the block on the funds following February's deal, the transfer of the cash was delayed because many international banks were wary of touching the money.

This meant that a round of multi-party talks in March ended without progress, and an April deadline to shut Yongbyon was ignored.

Movement on the issue finally came in June when Dalkombank, in Russia's far east, agreed to act as a conduit for the transfer of the money from Macau to Pyongyang.

The completed transfer of the funds was confirmed on 25 June - a day before the four-man IAEA team arrived in Pyongyang.

So has the crisis been solved?

Hardly. Despite optimism that Yongbyon could be shut within weeks, negotiators say they are at the start of a very long process and there is still a lot of hard work ahead.

Some of the most difficult questions have not yet been raised - such as whether North Korea has an undisclosed uranium programme, or what happens to any existing North Korean nuclear weapons.

And cynics point out that the February's agreement is similar to the 1994 Agreed Framework. That broke down after the US accused the North over the uranium programme, and the North accused the US of reneging on a deal to supply two light water nuclear reactors.

Although there are signs of a rapprochement between the US and North Korea, suspicion on both sides remain and there is huge potential for pitfalls and further delays.

Why does this issue matter so much?

North Korea's nuclear ambitions have the potential to become the most serious threat to East Asia's short and long-term security.

The two Koreas remain technically still at war, since no peace treaty was signed after the 1950-53 Korean war.

North Korea's relations with the South have improved markedly since 2000, but their border is still one of the most heavily militarised in the world, with thousands of artillery pieces aimed at the South Korean capital.

And North Korea's successful 2006 nuclear test greatly increased the risk of an East Asian arms race, as countries like Japan, South Korea and Taiwan were forced to weigh up whether to go nuclear as well.

What do we know about North Korea's nuclear weapons programme?

Since December 2002, when North Korea restarted its Yongbyon reactor and forced two UN nuclear monitors to leave the country, it has claimed to be working on building up its nuclear weapons arsenal.

The problem for the rest of the world is that it is very difficult to verify these claims.

If the Yongbyon reactor were fully operational, some analysts believe it could produce enough plutonium to build approximately one weapon per year.

America's CIA says a separate, enriched uranium programme could be producing "two or more" bombs each year by the middle of this decade, although North Korea has always publicly denied the programme's existence.

Experts believe that North Korea may have extracted sufficient plutonium for a small number of bombs.

US officials have put the number at "one or two".

About 8,000 spent fuel rods that were put into storage in 1994 could also be used to extract enough weapons-grade plutonium for a handful more weapons, the US believes.

Other estimates say the North may now have eight or more bombs.

Could North Korea now drop a nuclear bomb?

Although the North has tested a nuclear bomb, security analysts do not believe it has managed to make a device small enough to deliver on a missile.

This implies that for now at least, its only way of dropping a bomb would be via aircraft, which the US and its allies would be able to monitor.

However, the North is also working on long range missiles, one of which is believed to have a potential range of several thousand kilometres.

What is the background to the crisis?

Relations between the US and North Korea have been deteriorating since President George W Bush labelled North Korea part of an "axis of evil" in January 2002.

Tensions really started escalating the following October, when the US accused North Korea of developing a secret, uranium-based nuclear weapons programme.

Washington is not only concerned about the development of such weapons in North Korea, but also wants to curb Pyongyang's capacity to export missile and nuclear technology to other states or organisations.

It is often very difficult to tell what lies behind North Korea's moves. Pyongyang and its mercurial leader Kim Jong-il act in erratic and contradictory ways.

But it seems likely that North Korea has been trying to use the nuclear issue as a hard-line ploy to negotiate a non-aggression pact with the US and improved economic aid.