By Penny Spiller
North Korea's nuclear weapons programme has long been shrouded in secrecy - and its plans to test a nuclear bomb are the same.
All eyes are on North Korea and its possible test sites
The country has not said when, where or how any test might be carried out, so analysts have been left speculating on the possibilities.
South Korean monitors say they have detected no sign as yet of increased preparations for nuclear testing in the Communist nation.
But suspicion has already fallen on a number of sites long thought likely to be used in the event of a nuclear test.
They include Gilju in North Hamgyeong Province, which has been under South Korean and US observation since the 1990s when the construction of a pit was discovered in a nearby mountain valley.
South Korean intelligence officials last month reported increased activity around Mount Mantap, in the same province as Gilju. They claimed a 700m deep pit had been dug into the mountainside.
Another area of interest has been Hagap in the centre of the country, a facility comprising dozens of buildings at the foot of a mountain range with tunnel entrances.
One South Korean politician, who sits on the parliamentary defence committee, said on Wednesday that North Korea has eight underground tunnels it could use to carry out nuclear tests.
Such activities would suggest that any test will be carried out underground, but that is by no means a certainty, says Andreas Persbo of the London-based Verification Research, Training and Information Centre (Vertic).
He points to one study which found that North Korea's shallow ground water table would make it very difficult for an underground test to be carried out without polluting the country's water system.
Pyongyang is also not thought to be bound by any international treaties that would require it to refrain from a surface test.
But a lack of suitable spots in the mountainous country from which to launch any device, as well as the dangers of nuclear fall-out, makes a surface test less likely.
The greatest problem with an underground test will be some leakage, Professor John Simpson at the Mountbatten Centre for International Studies says. The site would need to be sealed after testing.
"It is also very difficult for outside countries to get a clear idea of what has been tested when it's underground," he says.
"With an above-ground test, you can sample the fall-out and get some idea of what they are up to."
He says that outside monitors trying to identify a test site will be looking for a number of things as well as, in the case of an underground test, a large hole in the ground.
They will be assessing whether an area's rock-type can be drilled into, whether there are buildings close by and if there is lots of wiring around the site.
"The obvious places are military bases or areas that are barren and sparse, away from populations," he says.
Andreas Persbo says it would not involve too much planning to prepare a nuclear test site.
But such preparations may be difficult to spot, particularly in a country as secretive as North Korea. Pakistan, he says, managed to camouflage what it was up to before its tests in 1998.
Signs are "trucks going in and out of a site, an increase in activity over the radio waves, or an increase in security."
But if North Korea does indeed test a nuclear weapon, the rest of the world will know about it very quickly, Mr Persbo adds.
The United States and South Korea have sophisticated monitoring devices to pick up activity in North Korea, and most seismographs around the world will be able to detect a test.
The Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) has 337 monitoring facilities around the globe.
As well as seismograph and infrasound networks, they have what are called radionuclide stations which would be able to detect any nuclear particles in the atmosphere.
Beneath the sea, a hydroacoustic network would be able to detect sound waves from any tests carried out near the coastline.
It remains to be seen whether such technology be needed in this case.