Q&A: North Korea nuclear programme
North Korea's nuclear programme remains a source of deep concern for the international community.
Since carrying out its third nuclear test in February 2013, North Korea has responded furiously to UN sanctions and joint US-South Korean military drills, culminating in a promise to restart facilities at its main Yongbyon nuclear complex.
The BBC looks at North Korea's nuclear ambitions and multi-national efforts to curtail them.
Has North Korea got the bomb?
Technically yes, but not yet the means to deliver it via a missile.
In 2006, 2009 and again in 2013, North Korea announced that it had conducted successful nuclear tests - they all came after the North was sanctioned by the UN for launching rockets.
Satellite data from P'unggye-ri, also known as P'unggye-yok, in a remote area in the east of the country, appeared to tally with claims that the experiments had been conducted underground.
Analysts believe the first two tests used plutonium as the fissile material. The North is believed to possess enough weapons-grade plutonium for at least six bombs.
Ever since North Korea warned that a third test would be a "high level" one, there has been speculation that it might involve a uranium device. It is so far unclear if this is the case.
Experts had previously said the state had not yet solved the problem of making a nuclear warhead small enough to fit onto a missile, but with the latest test North Korea claimed to have "miniaturised" a nuclear device.
That is yet to be independently verified.
What do we know about the North's nuclear programme?
The Yongbyon site is thought to be its main nuclear facility. The North has pledged several times to halt operations there and even destroyed the cooling tower in 2008 as part of a disarmament-for-aid deal.
But in March 2013, after a war of words with the US and with new UN sanctions for the North's third nuclear test, it vowed to restart all facilities at Yongbyon.
However, the US never believed Pyongyang was fully disclosing all of its nuclear facilities - a suspicion bolstered when North Korea unveiled a uranium enrichment facility at Yongbyon to US scientist Siegfried Hecker in 2010.
Mr Hecker's 2010 visit and subsequent report remains the most recent and reliable account of the complex.
While it appeared to be for electricity generation purposes, Mr Hecker said the facility could be readily converted to produce highly-enriched uranium for bombs. And with 1,000 centrifuges, he professed to be "stunned" by the sophistication of the plant.
Both the US and South Korea have also said that they believed the North had additional sites linked to a uranium-enrichment programme.
In August last year, satellite images of Yongbyon showed a light water reactor - which Pyongyang said was for civilian energy purposes - was nearing completion.
Experts say that reactor could be used to produce plutonium and the plant could be converted to produce highly enriched uranium for weapons.
The latest statement from the North talks about "altering nuclear facilities"- some analysts say this could mean a scenario where North Korea begins openly enriching uranium to weapons-grade levels.
A test based on a uranium device would spell new dangers for monitoring and proliferation because weapons-grade plutonium happens in large facilities that are easier to spot.
Uranium "enrichment" uses many, possibly small, centrifuges that can be hidden away. While North Korea has depleted its stocks of "reactor-grade" plutonium needed to make the weapons-grade variety, the country has plentiful reserves of uranium ore.
What has the international community done about the programme?
Multiple rounds of negotiations have taken place between the North, the US, Russia, China, Japan and South Korea aimed at persuading Pyongyang to give up its nuclear ambitions.
In September 2005, after more than two years of on-off talks, North Korea agreed a landmark deal to give up its nuclear ambitions in return for economic aid and political concessions.
But implementing the deal proved extremely difficult and the talks stalled in April 2009 over the issue of whether North Korea was fully disclosing its nuclear assets.
In July 2011, contact began again between the US and North Korea aimed at restarting the talks.
Less than six months later, North Korea's long-time leader Kim Jong-il died. He was succeeded by his son, the young Kim Jong-un.
In February 2012 North Korea suddenly announced it had agreed to suspend nuclear activities. It also said it was placing a moratorium on nuclear and long-range missile tests. Its reward would be food aid from the US.
But that deal was suspended following Pyongyang's 13 April 2012 unsuccessful rocket launch.
A successful rocket launch in December 2012 and a third nuclear test in February prompted the UN to further tighten sanctions.
What was the significance of the third nuclear test?
The test was a clear statement of North Korea's ongoing defiance of the international community. Pyongyang had warned that a "high-level" test was imminent, calling it a response to UN sanctions.
Its significance is likely to be vastly enhanced if it is confirmed that North Korea has indeed "miniaturised" a device, that is, made a device small enough to fit a nuclear warhead onto a missile.
This is likely to be of grave concern to the US and North Korea's neighbours.
Pyongyang also said the latest test had a much greater yield than the plutonium devices it detonated in 2006 and 2009.
Some analysts have suggested that warnings of a "high-level" test could have been code for the use of highly enriched uranium (HEU) rather than plutonium.
Although both represent roughly the same level of threat, a uranium bomb would signify a huge technological achievement - the process of distilling natural uranium ore to the stuff suitable for bombs is profoundly difficult.
The 2013 test was indeed larger in force than previous ones but monitors failed to detect radioactive isotopes, hampering efforts to fully assess the device. Eight samples had been analysed but nothing found, South Korea's Nuclear Safety and Security Commission said.
Finding certain isotopes - xenon gases in particular - would help experts determine whether a plutonium or uranium-based device was used.
But a well-contained test could yield no radioactive isotopes, experts say. It is still unclear whether the 2013 test used plutonium or uranium.