North Korea nuclear tests: what did they achieve?
North Korea has conducted four underground nuclear tests so far.
Each one has taken it closer to what decades of international talks have tried to prevent - a nuclear weapon in the hands of one of the world's most unpredictable states.
The four tests
9 October 2006 - a weapon for 'peace'
Years of posturing - and attempts at negotiation by foreign powers - culminated in October 2006 with an announcement by Pyongyang that it had carried out its first nuclear explosion.
Like all tests that would follow, it took place underground, in tunnels dug into a remote mountainous site called Punggye-ri, in the north-east.
The device is assumed to have used plutonium, sourced from the North's nuclear facility at Yongbyon.
International observers estimated the blast had an energy discharge of about a kiloton, less than a tenth of the size of the bomb dropped on Hiroshima in 1945.
Many believed this indicated a nuclear "fizzle" rather than a fully effective blast.
But the North said it had joined the nuclear club and that its bomb would contribute to peace and stability on the Korean peninsula.
25 May 2009 - attempt to prove stability
The second test was bigger, with an estimated yield of 2-8 kilotons.
North Korea said it had achieved a "higher level in terms of its explosive power and technology".
While the international consensus was that a nuclear test had happened, no radiation was detected. The ability to contain a nuclear test would in itself be a big advance for the North.
It also followed hard on the heels of a test of a missile with a long enough range to reach the US.
Both were seen as an attempt by ailing leader Kim Jong-il to prove the North's nuclear capacity before he died.
12 February 2013 - uranium-enrichment?
In the early hours of 12 February 2013, unusual seismic activity was again detected around Punggye-ri.
The North said it had tested "a miniaturised and lighter nuclear device with greater explosive force than previously".
The as-yet unproven reference to "miniaturised" stoked fears that Pyongyang was closer to producing a device small enough to fit on a long-range missile.
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.
6 January 2016 - thermonuclear claim
The first indication was again report of an "artificial quake" in North Korea, registering about magnitude 5.1, close to Punggye-ri.
North Korea later announced it had conducted its first successful test of a hydrogen bomb.
H-bombs, also known as thermonuclear warheads, are massively more powerful than atomic bombs, using fusion - the merging of atoms - rather than fission to unleash enormous amounts of energy.
Though again it has never been confirmed, that claim alarmed the international community.
Observations from afar suggested the blast detected was not large enough to have been a full thermonuclear device, but may have involved some nuclear fusion.
Again, North Korea said the device had been miniaturised.
9 March 2016 - a deliverable device?
A few months later, Kim Jong-un announced that North Korea scientists had been able to do what had long been feared, and make a nuclear warhead small enough to fit on a warhead.
It came after days of threats to carry out "indiscriminate" attacks on the US and South Korea. But analysts said it was impossible to confirm.