North Korea's nuclear test opens a rare, limited window for expert evaluation of its atomic weapons programme, with an added urgency lent by Pyongyang's claim to have detonated a "miniaturised" device.

Seismic monitors and "sniffer" planes capable of collecting radioactive evidence of Tuesday's test will provide the forensic material for analysts to try to determine the exact yield and nature of the underground explosion.

Pyongyang said the "high-level" test involved a "miniaturised and lighter atomic bomb" with a much greater yield than the plutonium devices it detonated in 2006 and 2009. Miniaturisation is needed to fit a warhead on a missile.

South Korea's defence ministry said seismic data suggested the explosive yield was significantly higher than the two previous tests at six to seven kilotons.

One key question analysts will be looking to answer was whether the North has switched from plutonium to a new and self-sustaining nuclear weaponisation programme using uranium.

Judging the type of fissile material requires the detection and analysis of xenon gases produced in the atomic explosion.

"These aren't necessarily easy to find and, if the test was well contained, may not be found at all," said Paul Carroll, programme director at the Ploughshares Fund, a global security foundation based in California.

"The miniaturisation claim is provocative because that's exactly the technology we don't want them to have," Carroll said, adding that it was a very difficult claim to confirm or refute.

The same six-seven kiloton yield could equally be achieved with a small, efficient device or a very large, inefficient one, with seismic data unable to differentiate between the two, he said.

Proof that the North had mastered warhead miniaturisation would be an alarming game changer -- especially given its successful rocket launch in December which marked a major step forward in ballistic prowess.

A uranium test would confirm what has long been suspected: that the North can produce weapons-grade uranium which doubles its pathways to building more bombs in the future.

A basic uranium bomb is no more potent than a basic plutonium one, but the uranium path holds various advantages for the North, which has substantial deposits of uranium ore.

"One alarm it sets off is that a uranium-enrichment programme is very easy to hide," said Mark Fitzpatrick, director of the non-proliferation programme at the International Institute for Strategic Studies in London.

"It doesn't need a reactor like plutonium, and can be carried out using centrifuge cascades in relatively small buildings that give off no heat and are hard to detect," he added.

North Korea revealed it was enriching uranium in 2010 when it allowed foreign experts to visit a centrifuge facility at its Yongbyon nuclear complex.

Many observers believe the North has long been enriching weapons-grade uranium at other secret facilities.

Another red flag raised by a uranium device relates to proliferation.

Highly enriched uranium is the easiest fissile material to make a crude bomb from, and the technical know-how and machinery for enriching uranium is more readily transferred and sold.

Scientist and nuclear expert Siegfried Hecker, who was among those shown the Yongbyon enrichment facility in 2010, had said a uranium test was the most likely scenario given Pyongyang's stated desire to boost its nuclear arsenal.

Pyongyang has a very limited plutonium stockpile -- enough Hecker estimates for four to eight bombs -- and it shut down its only plutonium source, a five-megawatt reactor at Yongbyon, in 2007.

Before Tuesday's test, Hecker had predicted that Pyongyang would claim total success and tout the device's sophistication.

"It will be difficult to distinguish truth from propaganda, but experience shows there is often a nugget of truth in North Korea's claims," he said.