How advanced is North Korea's nuclear programme?
North Korea's nuclear programme remains a source of deep concern for the international community. The BBC looks at North Korea's nuclear ambitions and multi-national efforts to curtail them despite three nuclear tests.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.
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. Whether it used plutonium or uranium for the 2013 test is unclear.
North Korea claimed it "miniaturised" a nuclear device for that test. It's hard to know if this is true - experts had previously said it could not make a nuclear warhead small enough to fit onto a missile.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 (purportedly for electricity generation) 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.
Both the US and South Korea have also said that they believed the North had additional sites linked to a uranium-enrichment programme.
A test based on a uranium device would spell new dangers for monitoring and proliferation because weapons-grade plutonium enrichment 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. But none of this has ultimately deterred North Korea.
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.
Implementing the deal proved extremely difficult and the talks stalled in April 2009.
Contacts in July 2011 did not get far before long-time leader Kim Jong-il died to be succeeded by his son, Kim Jong-un.
Another step forward in early 2012, when North Korea suddenly announced it would suspend nuclear activities and place a moratorium on missile tests in exchange for US food aid, came to nothing when Pyongyang tried to launch a rocket in April that year.
The UN further tightened sanctions after the 2013 test.Did the 2013 test advance North Korea's nuclear capabilities?
North Korea made certain claims about its capabilities in the wake of its test.
Firstly, it claimed it had "miniaturised" a device, that is, made a device small enough to fit a nuclear warhead onto a missile.
If true, 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 because 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. Finding certain isotopes 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. So uncertainty remains.