North Korea's nuclear programme: How advanced is it?
North Korea's nuclear programme remains a source of deep concern for the international community. Despite multiple efforts to curtail it, Pyongyang says it has conducted five nuclear tests.
Has North Korea got the bomb?
Technically yes - North Korea has conducted several tests with nuclear bombs.
However, in order to launch a nuclear attack on its neighbours, it needs to be able to make a nuclear warhead small enough to fit on to a missile.
North Korea claims it has successfully "miniaturised" nuclear warheads - but this has never been independently verified, and some experts have cast doubt on the claims.
How powerful are North Korea's nuclear bombs?
North Korea says it has conducted five successful nuclear tests: in 2006, 2009, 2013 and in January and September 2016.
The yield of the bombs appears to have increased.
September 2016's test has indicated a device with an explosive yield of between 10 and 30 kilotonnes - which, if confirmed, would make it the North's strongest nuclear test ever.
The other big question is whether the devices being tested are atomic bombs, or hydrogen bombs, which are more powerful.
H-bombs use fusion - the merging of atoms - to unleash massive amounts of energy, whereas atomic bombs use nuclear fission, or the splitting of atoms.
The 2006, 2009 and 2013 tests were all atomic bomb tests.
North Korea claimed that its January 2016 test was of a hydrogen bomb.
But experts cast doubt on the claim given the size of the explosion registered.
Details of the fifth test have not yet been released.
Plutonium or uranium?
Another question is what the starting material for the nuclear tests is.
Analysts believe the first two tests used plutonium, but whether the North used plutonium or uranium as the starting material for the 2013 test is unclear.
A successful uranium test would mark a significant leap forward in North Korea's nuclear programme. The North's plutonium stocks are finite, but if it could enrich uranium it could build up a nuclear stockpile.
Plutonium enrichment also has to happen in large, easy-to-spot facilities, whereas uranium enrichment can more easily be carried out in secrecy.
Can it deliver it?
There is no consensus on exactly where North Korea is in terms of miniaturising a nuclear device so that it can be delivered via a missile.
In March 2016 Pentagon spokesman Peter Cook said the US had not seen North Korea demonstrate an ability to miniaturise a warhead. Two days later Adm Bill Gortney, the officer responsible for defending US air space, told a Senate panel it was "prudent" to assume that Pyongyang could strike the US, despite the intelligence community giving it "a very low probability of success".
Prof Siegfried S Hecker of Stanford University, a highly authoritative voice on North Korea's weapons' development, says "we must assume that the DPRK has designed and demonstrated nuclear warheads that can be mounted on some of its short-range and perhaps medium-range missiles".
Writing in September 2016, he said Pyongyang's ability to field an intercontinental ballistic missile fitted with a nuclear warhead capable of reaching the US was "still a long way off - perhaps 5 to 10 years, but likely doable if the programme is unconstrained".
What else do we know about the North's nuclear programme?
A site in the mountains near Yongbyon, north of Pyongyang, is thought to be the North's main nuclear facility, while the January and September 2016 tests were said to have been carried out at the Punggye-ri site.
The Yongbyon site processes spent fuel from power stations and has been the source of plutonium for North Korea's nuclear weapons programme.
Both the US and South Korea have also said that they believe the North has additional sites linked to a uranium-enrichment programme. The country has plentiful reserves of uranium ore.
What has the global community done about this?
The US, Russia, China, Japan and South Korea engaged the North in multiple rounds of negotiations known as six-party talks.
There were various attempts to agree disarmament deals with North Korea, but none of this has ultimately deterred Pyongyang.
In 2005, North Korea agreed to a landmark deal to give up its nuclear ambitions in return for economic aid and political concessions.
In 2008, it even destroyed the cooling tower at Yongbyon as part of the disarmament-for-aid deal.
But implementing the deal proved difficult and talks stalled in 2009.
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.
Then in 2012, North Korea suddenly announced it would suspend nuclear activities and place a moratorium on missile tests in exchange for US food aid.
But this came to nothing when Pyongyang tried to launch a rocket in April that year.
In March 2013, after a war of words with the US and with new UN sanctions over the North's third nuclear test, Pyongyang vowed to restart all facilities at Yongbyon.
By 2015, normal operations there appeared to have resumed.
China's reaction to the tests has been closely watched - as it is the North's main trading partner, and only ally.
The 2016 tests brought international condemnation, including from China, and increased sanctions.
However, Beijing has generally been anxious not to do anything to destabilise its volatile neighbour.