North Korea has vowed to carry out a third nuclear test, but scientists and concerned foreign governments may have a tough time verifying the actions of the reclusive state.

One critical question is whether North Korea uses uranium or plutonium. North Korea's 2006 and 2009 tests involved plutonium, so a uranium detonation would prove that Kim Jong-Un's regime has opened an additional way to make bombs.

After foreign detection of its 2006 test, North Korea carried out its 2009 explosion deep underground, which gave scientists far less information. Most observers expect North Korea to be even more careful this time.

Some 270 stations and laboratories around the world monitor seismic and other activity under the Comprehensive Test Ban Treaty Organization, making up about 80 percent of the total envisioned under the 1996 UN treaty.

EARTHQUAKE DETECTION: Seismic monitoring is the most effective and quickest way to detect a nuclear test. Seismic waves travel about eight kilometers (five miles) per second.

Seismic detention put the May 25, 2009, test at around 4.5 magnitude with an explosive yield of a few kilotons, well below that of the nuclear bombs which the United States dropped on Japan in 1945. North Korea's 2006 test was detected at magnitude 4.1.

URANIUM OR PLUTONIUM?: Both uranium and plutonium can be used in nuclear weapons. The United States devastated Hiroshima with a uranium bomb and Nagasaki with a plutonium bomb.

North Korea's nuclear program was historically based on plutonium, but the regime disclosed in November 2010 to visiting US scientists that it was operating a uranium enrichment plant.

Experts have speculated that North Korea may have decided on a third test in hopes of trying out its uranium program or that it may simultaneously use both methods. But if North Korea prevents leakage, it may be impossible to know for sure.

A successful test with highly enriched uranium would alarm North Korea's adversaries as it is much easier to conceal work with uranium than with plutonium, which requires a reactor to produce the chemical element.

RADIONUCLIDE SIGNALS: This extremely sensitive technique, which could be crucial in determining the nature of a North Korean test, allows scientists to examine material that has seeped out of the ground or been released in the air.

Scientists use instruments that "sniff" fission products of the explosive material and then use modeling to determine the origin of the radionuclides and predict where the nuclear plume may be headed.

Two weeks after the 2006 test, the isotope Xenon-133 was detected across the Pacific Ocean in the northern Canadian city of Yellowknife. But the North Koreans were successful in 2009 in sealing off the test site.

INFRASOUND DETECTORS and HYDROACOUSTIC TECHNOLOGY: Undetectable to the human ear, infrasound waves have frequencies between 0.01 and 10 Hz. They are typically produced by explosions in the atmosphere but can also come from underground explosions.

A very small infrasound signal was detected following the 2009 North Korea test, but none in 2006.

Hydroacoustic technology can be used to detect nuclear explosions in or near bodies of water by tracking sound waves.

Global monitors can detect an in-water explosion as small as one ton (0.001 kiloton) across most of the world's oceans.

US PLANES, SATELLITES

The US Air Force was first tasked in 1947 with monitoring atomic explosions worldwide. Nearly 1,000 personnel work at the Air Force Technical Applications Center, based in Florida near the Kennedy Space Center.

AFTAC operates a WC-135 aircraft for detecting radioactive debris that could come from nuclear explosions. The plane flies to the location of the debris plume and collects particulates for laboratory analysis.

The United States also uses satellites to detect potential nuclear explosions in space or in the atmosphere -- a capacity that is not part of the Comprehensive Test Ban Treaty Organization.

Satellites can collect data on electromagnetic pulses, optical flashes and nuclear radiation.

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.