Condemnation followed from Seoul, but the timing may have been chosen to constrain an effective policy response: the Lee Myung-bak government leaves office on February 25, to be replaced by Park Geun-hye.

Although its exact timing was unknown, the test was widely expected as North Korea seems to be operating a roughly three-year cycle of missile and nuclear tests. Previous missile tests took place in 2006, 2009 and on December 12; previous nuclear tests happened in 2006 and 2009.

Moreover, following UN Security Council condemnation of its December satellite launch - widely believed to be cover for an intercontinental ballistic missile test - Pyongyang media had warned that it would conduct a "higher level" nuclear test.

Experts are awaiting evidence of whether the device was plutonium-based, like both previous tests, or uranium-based. North Korea's plutonium reactors are shut down, and Pyongyang is believed to have extracted only enough plutonium for about 12 devices, but is believed to have run a uranium-based weapons programme since around 2000.

Uranium offers greater potential to create weapons-grade material and is easier to proliferate.

"If it is uranium-based, that means they have the capability to enrich uranium to bomb grade in significant quantities," said Dan Pinkstone, who heads the International Crisis Group's Seoul office. "And uranium enrichment is much easier to conceal - it is more difficult to detect clandestine facilities - so if you are going to sell it to a terrorist group, that is much more appealing."

North Korea is determined to possess nuclear arms and intercontinental ballistic missiles, regardless of external pressures.

In the world's most militarised society, these most powerful arms are seen as security against Pyongyang's perceived external threat, the United States: a favoured motif in North Korean propaganda posters is missiles raining down upon Washington, and the state PR arm recently released a video clip in which New York goes up in flames.

But the programmes also constitute a hedge against potential internal threats: a coup by a disaffected faction within the military or a rebellion by impoverished locals. Either group may call in external allies.

"North Korea wants nuclear weapons because they believe that then external forces could not attack them," said Shin Ju-hyun, chief editor of Seoul-based information source DailyNK, which employs North Korean defectors. "The second reason is that North Korean people and officials will believe that their regime will never fall, so will never do anything against the regime."

Recent Middle Eastern events may have accelerated Pyongyang's efforts.

"They saw what happened to Colonel Gaddafi, who was given aid packages and preferential treatment in world markets in exchange for giving up nuclear weapons," said Andrei Lankov, a North Korea expert at Seoul's Kookmin University. "He was overthrown in a popular revolt that would have had less chance to succeed without outside help, and had he been in possession of nuclear weapons, other counties would have been far more cautious about getting involved in Libya's civil war."

The success of both the December missile test and yesterday's nuclear test is likely to boost the standing of Kim Jong-un among the power elites in North Korea.

"Kim Jong-un, compared to [his late-father and predecessor] Kim Jong-il, has not yet got a full grasp of power," Shin said. "He is still in the process of solidifying and stabilising his power."

How the international community can respond is unclear. North Korea can target Japan and South Korea with a range of conventional and possibly nuclear weapons, and maintains a 1.2 million strong military (nearly double the size of South Korea's). The Sparta-like state means a military solution is not viable.

Experts say new sanctions could target specific North Korean companies and financial institutions, and the Proliferation Security Initiative, under which vessels are boarded on the high seas to search for illegal cargoes, could be aggressively applied to North Korea. But it is already the most heavily sanctioned nation on earth.

Only one nation has significant leverage against North Korea: China. Following North Korea's first nuclear test in 2006, China did cut off fuel to North Korea - but only for a few days.

This reluctance to apply pressure is based on a wider strategic imperative facing Beijing.

Kim's state remains a critical strategic buffer against South Korea, Japan and the US guarding China's northeastern flank.

The best possibility now may be for the international community to rethink the aims of the North Korean regime, which wants to be accepted as a de facto member of the nuclear club the way India and Pakistan are.

A strategic recalibration based on this reality - under which North Korea may accept a cap on future programmes and an agreement not to proliferate, in return for economic aid and diplomatic recognition - may be preferable to what one expert calls the "fantasy" of North Korean denuclearisation.

"I believe this fantasy should have been given up many years ago, but the behaviour of big players is not always rational," Lankov said. "I believe we will see another round of meaningless efforts to achieve the unachievable - the denuclearisation of North Korea, and only if and when the next round fails will there be serious consideration about acceding to the obvious."

Key dates on the road to making a bomb

October 16, 2002 The United States announces that North Korea admitted having a clandestine programme to enrich uranium for nuclear weapons.

December 12, 2002 North Korea tells the International Atomic Energy Agency it is restarting its one functional nuclear reactor and reopening other facilities.

January 10, 2003 North Korea announces that is has withdrawn from the nuclear Non-Proliferation Treaty.

August 27-29, 2003 The first round of six-party talks is held in Beijing, grouping China, Japan, Russia, the United States and the two Koreas.

February 10, 2005 North Korea's foreign ministry announces that Pyongyang has produced nuclear weapons.

September 19, 2005 Six-party talks deal commits North Korea to abandoning all nuclear weapons and programmes, and returning to the NPT.

July 4-5, 2006 North Korea test-fires seven ballistic missiles, including a failed test of its longest-range missile, the Taepo Dong-2

July 15, 2006 UN Security Council resolution condemns the missile launches.

October 9, 2006 North Korea conducts first nuclear test underground.

October 14, 2006 UN Security Council imposes fresh sanctions on Pyongyang.

July 16, 2007 The IAEA confirms the shutdown of facilities at Yongbyon.

October 2-4, 2007 Second North-South Korea summit.

April 5, 2009 North Korea launches three-stage Unha-2 rocket.

April 13, 2009 UN Security Council presidential statement condemns rocket launch.

April 14, 2009 North Korea withdraws from six-party talks.

April 16, 2009 North Korea ejects IAEA and US monitors.

May 25, 2009 North Korea conducts its second underground nuclear test. June 12, 2009 UN Security Council resolution expands sanctions on Pyongyang.

March 26, 2010 The South Korean patrol ship Cheonan is sunk near maritime border with North. Inter-Korean engagement frozen.

November 12, 2010 North Korea reveals uranium enrichment facility to visiting foreign experts.

December 17, 2011 North Korean leader Kim Jong-il dies and is succeeded by his youngest son, Kim Jong-un.

December 12, 2012 North Korea successfully launches the Unha-3.

January 22, 2013 UN Security Council resolution expands existing sanctions.

January 24, 2012 The North announces its intention to conduct another nuclear test and continue rocket launches.

February 12, 2013 Pyongyang detonates a nuclear device, with regional monitors detecting an unusual seismic event, of a magnitude between 4.9 and 5.1, in the same location as the North's Punggye-ri nuclear test site. Seoul estimates the blast yield at six to seven kilotons.

Agence France-Presse