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North Korea carried out an apparent nuclear test Tuesday, its third, despite numerous appeals and warnings to desist.
Here are some questions and answers about the country's nuclear weapons programme, which has grown over the decades despite ever-tighter UN sanctions:
Q. Is North Korea a nuclear weapons state?
A. No, or at least not yet. It conducted confirmed nuclear tests in 2006 and 2009. Both were believed to be fairly crude plutonium devices with relatively low yields, and the 2006 test was widely seen as a dud.
So, in one sense, it does have the bomb. But it has no proven missile delivery system and, crucially, no proven ability to shrink a nuclear device to fit a missile warhead.
Insofar as it has a nuclear weapon, it is one that could only be delivered by plane, boat or truck.
Q. What about December's rocket launch?
A. The December launch marked a major step in the North's development of an intercontinental ballistic missile, but numerous technical hurdles remain including, most importantly, re-entry technology.
The December rocket showed it could place an object in orbit, not bring it to Earth again.
Most analysts estimate North Korea is still years from developing a genuine ICBM capacity.
Q. What was expected from the third test?
A. North Korea had promised it will be a "higher-level" test, which led to speculation that it might involve a uranium device, or possibly a simultaneous detonation of separate uranium and plutonium devices.
On Tuesday a seismic event was measured in the area of the North's test site, which South Korea said was a blast measuring six to seven kilotons.
Experts estimate the North has been secretly enriching uranium to weapons-grade level for years.
Seismic and atmospheric fallout from the test will be closely monitored and analysed for clues, but a well-contained underground test would provide scant material for confirming or refuting North Korean claims.
Q. Where has the apparent test been held?
A. The 2006 and 2009 tests were conducted at the Punggye-ri nuclear test site, in a remote mountainous region in the northeast, about 100 kilometres (60 miles) from the border with China and 200 km from the Russian border.
Tuesday's seismic event was in the same area.
The site comprises three test tunnels dug into the granite bedrock of a 2,200-metre (7,220-foot) mountain. They are known in the South Korean media as the east tunnel (used for 2006 test), west tunnel (2009) and the newest south tunnel.
Q. What will be the impact of a third test?
A. Any type of North Korean nuclear test is a trigger for global concern. Soon after reports of Tuesday's blast, diplomats said the UN Security Council would convene.
The UN resolution condemning the December rocket launch warned of "significant action" if the North proceeded with a nuclear test.
The level of concern will vary according to the perceived size and sophistication of the test.
Any confirmation that the North can produce weapons-grade uranium will ring a number of alarm bells.
With its tiny plutonium stockpile capped, uranium offers the best way for Pyongyang significantly to boost its atomic weapons stockpile. Uranium enrichment facilities are also difficult to detect.
A third test would likely scupper any hope of an early resumption of six-party talks on North Korea, involving the two Koreas, China, Japan, Russia and the United States.
Given the absence of a delivery system, the test will not dramatically alter the regional strategic military balance in the short term.
But analysts say the further the North progresses with its weaponisation programme, the harder it will be to persuade it to give it up.