North Korea's expected atomic test will offer a rare chance to gauge where its nuclear programme is headed, with most expert attention focused on what type of device is detonated and how.

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

North Korea has been signalling that a test is imminent, and speculation of a major advance has been fuelled by the assertion of its top military body, the National Defence Commission, that any test will be of a "higher-level".

Numerous analysts believe this could point to the first-time test of a uranium device. The North's two previous tests in 2006 and 2009 both used plutonium for fissile material.

"It's not that a uranium test would reflect any great technical achievement," said Mark Fitzpatrick, director of the non-proliferation programme at the International Institute for Strategic Studies in London.

"But it 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," Fitzpatrick said.

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 Fitzpatrick.

"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, according to Paul Carroll, programme director at the Ploughshares Fund, a global security foundation based in California.

"Highly enriched uranium is the preferred currency of rogue states or terrorist groups," Carroll said.

"It's the easiest fissile material to make a crude bomb out of and the technical know-how and machinery for enriching uranium is more readily transferred and sold," he added.

Scientist and nuclear expert Siegfried Hecker, who was among those shown the Yongbyon enrichment facility in 2010, believes a uranium test is 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.

In an analysis published Tuesday in Foreign Policy magazine, Hecker held out the prospect of a multiple test in which separate plutonium and uranium devices are detonated simultaneously.

Digging into its precious stockpile to conduct a third plutonium detonation -- like the 2006 and 2009 tests -- would suggest Pyongyang had a miniaturized design that it was confident enough to try out.

A plutonium bomb requires less fissile material than a uranium version, making it easier to shrink to the size of a warhead that fits on a missile.

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

"Pyongyang will almost certainly claim that the test was successful and will tout its sophistication," Hecker said.

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

To some extent, Pyongyang has already boxed itself a corner by trumpeting the "higher-level" scenario.

"While this statement is usefully ambiguous, it does set a baseline of expectation," said Hugh Chalmers, a nuclear research analyst at the Royal United Services Institute.

"If a new test turns out like the 2006 test, they will be very disappointed," Chalmers said.

The 2006 detonation was widely judged a failure by the international community with a yield estimated at less than one kiloton.

Key dates in North Korea nuclear programme
Seoul (AFP) Feb 7, 2013 - Ahead of a widely expected nuclear test by North Korea, here are key dates in its nuclear and missile programmes:

1985

December 12: North Korea accedes to the nuclear Nonproliferation Treaty (NPT).

1992

January 30: North Korea concludes a comprehensive safeguards agreement with the UN's International Atomic Energy Agency (IAEA).

1993

February 9: The IAEA demands special inspections of two sites believed to store nuclear waste. North Korea refuses.

1994

June 13: North Korea announces its withdrawal from the IAEA.

July 9: North Korean President Kim Il-Sung dies and is succeeded by his son, Kim Jong-Il.

October 21: The United States and North Korea adopt the "Agreed Framework". The North commits to freeze and eventually eliminate its nuclear facilities in exchange for two light water reactors (LWR) and heavy-fuel supplies.

1998

August 31: North Korea launches a three-stage rocket called "Taepo Dong-1" with a range of 1,500-2,000 kilometres (930-1,240 miles) that flies over Japan.

1999

September 7-12: North Korea agrees to a moratorium on testing long-range missiles.

December 15: Five years after the Agreed Framework, contract signed to begin construction of two LWRs in North Korea.

2000

June 15: First-ever leaders' summit between North and South Korea. US relaxes sanctions on Pyongyang.

October 24: US Secretary of State Madeleine Albright meets Kim Jong-Il in Pyongyang.

2002

January 29: US President George W. Bush characterises North Korea as part of an "axis of evil" along with Iraq and Iran.

October 16: The United States announces that North Korea admitted having a clandestine programme to enrich uranium for nuclear weapons.

November 14: Heavy-fuel oil deliveries to North Korea are suspended.

December 12: North Korea tells the IAEA it is restarting its one functional reactor and reopening other facilities frozen under the Agreed Framework.

December 22-24: North Korea cuts all seals and disrupts IAEA surveillance equipment on its nuclear facilities and materials.

December 27: North Korea orders IAEA inspectors out of the country.

2003

January 10: North Korea announces its withdrawal from the NPT.

February 27: US officials confirm North Korea has restarted a five-megawatt nuclear reactor.

August 27-29: The first round of six-party talks is held in Beijing, grouping China, Japan, Russia, the United States and the two Koreas.

November 21: Suspension announced on construction of two LWRs.

2005

February 10: North Korea's foreign ministry announces that Pyongyang has produced nuclear weapons.

September 19: Six-party talks deal commits North Korea to abandoning all nuclear weapons and programmes, and returning to the NPT.

2006

July 4-5: North Korea test-fires seven ballistic missiles, including a failed test of its longest-range missile, the Taepo Dong-2.

July 15: UN Security Council resolution condemns the missile launches.

October 9: North Korea conducts first nuclear test underground.

October 14: UN Security Council imposes fresh sanctions on Pyongyang.

2007

February 13: China announces deal under which North Korea will disable nuclear plants at Yongbyon and let IAEA inspectors return. It will get one million tonnes of fuel aid and be removed from a US list of terrorist states.

July 16: The IAEA confirms the shutdown of facilities at Yongbyon.

October 2-4: Second North-South Korea summit.

2009

April 5: North Korea launches three-stage Unha-2 rocket.

April 13: UN Security Council presidential statement condemns rocket launch.

April 14: North Korea withdraws from six-party talks.

April 16: North Korea ejects IAEA and US monitors.

May 25: North Korea conducts its second underground nuclear test.

June 12: UN Security Council resolution expands sanctions on Pyongyang.

2010

March 26: The South Korean patrol ship Cheonan is sunk near maritime border with North. Inter-Korean engagement frozen.

November 12: North Korea reveals uranium enrichment facility to visiting foreign experts.

2011

December 17: North Korean leader Kim Jong-Il dies and is succeeded by his youngest son, Kim Jong-Un.

2012

April 13: North Korea attempts to launch a satellite using the Unha-3, but the rocket falls apart after take-off.

December 12: North Korea successfully launches the Unha-3.

2013

January 22: UN Security Council resolution expands existing sanctions.

January 24: The North announces its intention to conduct another nuclear test and continue rocket launches.