Mr Kim was incarcerated in an underground cell, designed, he said, so that people at ground level would not hear screaming. For weeks, he was beaten with timber, suffering a fractured skull and the loss of all his teeth. But the most agonising torture was 'the pigeon'. In an underground chamber too low to stand up in his hands were handcuffed behind his back and he was hung up for 10 hours at a stretch. 'I could not stand up or sit down. The muscles on the shoulders were paralysed and the bones seemed to break through the chest,' he recalled.

Two other prisoners in the underground cells - which had no sanitation facilities and no guards (the emaciated prisoners were left totally without care) - at the same time died. Under torture and deprived of food, Mr Kim finally confessed to the charge. Only then, with no judicial procedure, was he dispatched for a three-year sentence to Re-education Centre No15 - Yodok.

In 1999, Kim Eun-cheol, 26, was captured by police after escaping to Russia and then China. He was tortured by National Security Agency officials. For months, he was forced to kneel on a hot iron plate and beaten. However, he did not confess until he, too, was subjected to the pigeon. In 2000, he was sentenced to three years in Yodok.

The labour camp, in the North's mountainous centre, is a huge facility where political prisoners - and in some cases, their families - are sent for crimes such as criticising the system after studying abroad and attempting to defect to South Korea. Estimates of the numbers imprisoned there vary from 20,000 to 50,000.

The camp gained notoriety in the west after the publication of defector Kang Chol-hwan's The Aquariums of Pyongyang. In 2005, Mr Kang was invited to Washington to meet one of his readers, George W. Bush. It is also the setting of a grim, South Korea-produced musical entitled Yodok Story.

Inmates sent there soon find that although the physical brutality by the 1,000 or so guards at the camp is not as severe as that meted out by interrogators at security agencies, those who fall out of favour with the agents face a slow death.

'In Yodok, there is a way of killing prisoners without using violence,' said Mr Kim Gwang-soo. 'That is starvation, which is considered legal. The guards assign inmates targeted for killing hard work that they can never finish.'

Full rations, of about 600 grams a day, are only given after completion of labour assignments. The system turns prisoners against each other. If work is unfinished, they receive half-rations. 'There is battery among the prisoners themselves if you are not working your best,' said Mr Kim Eun-cheol.

There are mines and farms in the camp, as well as construction sites. The younger Mr Kim's work schedule involved waking at 4.30am for agricultural labour such as carrying fertiliser. Breakfast was at 7am, before construction work that went on until 6pm or 7pm with 30-minute meal breaks. Work units were often forced to labour late into - or through - the night. The facilities they built stood empty.

Anyone who criticises the supervisors (even to other prisoners - snitches are common) is sent to a separate area of the camp where, as Mr Kim put it, 'death came easily'. A meal there would consist of only 10 grams of food. The effect on those who survived a month was appalling. 'Someone more than 1.7 metres tall would actually end up as bent and as short as a child, and would barely survive another three days,' he said.

Inmates who died were buried at night, in unmarked, shallow graves in the 'grave valley'. Prisoners assigned to burial details would finish the job as quickly as possible, in order to spend time finding anything edible in the valley.

Even after release, prisoners are not free. On leaving the camp, Mr Kim was ordered to sign a pledge reading, 'I will face execution if I reveal the secrets of Yodok.'

Even so, rumours about the camp circulate: after his release, people in Mr Kim's work unit (he was a farm labourer) would ask how he had survived his time there. Remembering his pledge, and terrified that officials would learn what he had revealed, he never answered. He did not even dare inform his siblings about his experiences.

'Even in the safety of South Korea, I still have recurring nightmares about being thrown into Yodok,' said Mr Kim Gwang-soo. 'It's not easy for me to fall asleep without alcohol.'

Beyond common criminals, the US Committee on North Korean Human Rights estimates that up to 200,000 prisoners are held in political camps in North Korea. Although Yodok is not North Korea's only camp for political prisoners, it is the only one where non-life term sentences are served. Only one defector claims to have escaped from a life-term political camp.

North Korea has a massive state security apparatus. 'One security agent covers three apartment buildings,' said Mr Kim Gwang-soo. 'He has about 20 informants in the district, so one person oversees every three people.'

Conditions in the North Korean penal system might have improved since the two Kims' experiences, according to Citizens' Alliance for North Korean Human Rights (NKHR), a Seoul-based civic group. The group interviewed 20 defectors on their experiences in interrogation centres and camps, and last month published a booklet, North Korea: Republic of Torture. It has been distributed in 23 countries.

Pyongyang signed up to the International Covenant on Civil and Political Rights in 1981 and joined the UN in 1991. In 2003, the UN passed its first resolution against North Korea's human rights violations. In April 2004, the country revised its criminal code, including articles against the use of torture and the rights to fair trial and defence.

'We cannot confirm certain types of torture continue,' said NKHR programme officer Joanna Hosaniak. 'The pigeon torture does not appear in testimonials after 2002, and forced abortion has not been reported since 2000.'

NKHR research also indicates that those who escape from North Korea to China or Russia are not as severely punished as was the case in the 1990s. 'We heard that Kim Jong-il instructed that there be more moderate punishment for border- crossers, and ordered officials not to allow 'pumping', as that would lead them to try to escape again,' Ms Hosaniak said.

Pumping is a common practice by police and state security officials in which prisoners are stripped naked then forced repeatedly to sit and stand, sometimes hundreds of times, to force their bodies to deposit anything hidden.

But NKHR warns that its findings are tentative, and a report by Human Rights Watch in March concluded that punishment against defectors had stiffened since 2004.

Other sources say North Korea's border is now more heavily guarded than before. An American pastor and member of Catacombs, a Seoul-based Christian group, reported that on a recent trip to Northeast China, he saw North Korean border patrols with dogs and sniper rifles.

For now, NKHR urges foreign governments to engage North Korea while pressing it on human rights and offering help to upgrade its judicial and penal systems. The group supports the approach taken by Britain, which has diplomatic representation in Pyongyang and which has assisted North Korea in areas such as education, while applying public pressure on human rights, notably during the visit of Foreign Office Minister Bill Rammell in September 2004.

In December, NKHR is planning a conference on North Korean human rights in London and plans to invite the North Korean ambassador to speak.

The group has less praise for Seoul. 'We can do more,' said Lee Young-hwan, who wrote NKHR's March report. 'We think the South Korean policy is good in some aspects, but they should take the initiative to promote North Korean human rights.' But he also said: 'South Korea's whole paradigm on human rights is not secure yet. It is not so long since we got our own democracy.'

Despite the testimonials of defectors, some liberals in South Korea, sceptical of past disinformation about the North spread by the military governments of the 1960s, 70s and 80s, downplay tales of torture or even deny them.

Mr Kim Gwang-soo said: 'I would say to such a person, 'Go and live in North Korea'. It is not right for a person who has not lived in North Korea to distrust anything I say.'