A female North Korean soldier looks out from behind a barded wire fence around a camp on the North Korean river banks

Lim Hye-jin still shivers at the memory of two brothers who managed to escape briefly from her massive concentration camp in the mountains of North Korea. Seven family members were killed on the spot in revenge. Scores more prisoners were savagely beaten as collective punishment for the breakout.

Several weeks later, guards and inmates were ordered to gather as the pair – their bodies battered from torture – were dragged back behind the barbed wire. They had been caught in China and returned to the repressive regime.

'The two brothers were beheaded in front of everyone,' said Lim. 'They called everyone to watch as a warning not to flee. The other prisoners then had to throw stones at them.'

The scene left Lim, then 20, so traumatised she could not eat for days. Yet it was just one of many terrible incidents she saw during seven years as a camp guard, including routine killings, torture and rape of political prisoners declared enemies of the state.

One woman was stripped naked, then casually set on fire after annoying a guard during interrogation. 'They do not see them as human beings, just as animals,' said Lim.

Now Lim – who ended up in prison herself after being caught trading in China – has spoken exclusively about her experiences, offering a unique insight into the horrors of North Korea's forced labour camps holding an estimated 200,000 people.

Her revelations come as tensions continue to rise on the Korean peninsula. US President Donald Trump is pressing China to act over its cruel neighbour with warnings of possible 'major conflict' if the stand-off over North Korea's attempts to develop nuclear weapons is not resolved through diplomacy.

Lim is the first female guard to talk openly about her experiences.

'We were manipulated not to feel any sympathy for prisoners,' she said. 'We were told they had committed terrible crimes. Now I know they were normal people so I feel very guilty.'

Few have escaped these hidden hellholes, modelled on Stalin's gulags and compared to those run by the Nazis. Even children are incarcerated for life, along with parents and grandparents, under rules that punish three generations for perceived dissidence.

One defector told me he was the only one of 5,000 children to escape his camp, which held close to 50,000 people. Many prisoners are stunted and deformed by hunger and back-breaking labour in freezing forests and deep mines. Former inmates told me of living in fear of constant beatings, of injured people dumped to die in the snow, of hundreds sealed beneath ground after mining accidents, of rotting corpses piled beside huts, of catching snakes to survive deadly starvation.

This is a rare photograph of some of the prisoners who ended up inside one of the camps in North Korea

Satellite evidence suggests some of these barbaric units have grown since Kim Jong Un took over the family dictatorship six years ago, although their existence is denied.

Camp 12, a fenced-off farm growing corn and peppers near the Chinese border, was where Lim first began working, aged 17. Inmates included high-ranking officials who had fallen out of favour with the regime.

Guards, brought up in a system that deifies the Kim dynasty, were given brainwashing sessions twice a week and told not to see prisoners as humans. 'Even if a guard was driving and ran over a child, there would not be real punishment,' she said.

Most inmates at the two camps where she worked were women and children. 'If men were healthy they would be sent to the mines where they were used as disposable labour. Many died. They were also made to psychologically suffer.'

One survivor told me of frequent accidents in quarries and mines as exhausted inmates worked round the clock. 'On one occasion, 300 people lost their lives in a gas explosion. The guards just closed off the tunnels with others trapped inside to stop the fires and gas spreading.'

Hundreds are also alleged to have died hollowing out huge tunnels in Mount Mantap for the testing of nuclear weapons and then clearing the contaminated sites afterwards.

Lim said male guards abused women in camps by having what they called 'affairs' with them. 'It was basically rape because prisoners did not have the right to say no.'

If women became pregnant, they had to have abortions or were killed by lethal injection, and if pregnancy was too advanced, babies were beaten to death or buried alive.

Camps have 'reward marriages', in which model prisoners are given a mate selected by camp chiefs as an incentive to work hard. Men and women are otherwise kept apart.

This is a satellite image of Camp 12, a fenced-off farm growing corn and peppers near the Chinese border, and was where Lim first began working, aged 17

Prisoners work seven days a week, are woken at 5am and spend up to 16 hours slaving away in fields and factories before evening 're-education' sessions at which they might be made to memorise official edicts. Failure means being kept awake all night. Inspections take place three times a day to check no one has fled through three rings of barbed wire surrounding the camps and anyone out of their dormitory at night is shot.

Lim said even after death, prisoners were denied humanity. 'All the bodies were piled up to one side. There was no respect, no funeral process. After a week the corpses would be burned.'

Wearing ragged ex-army uniforms and sandals made from tyres, inmates suffer starvation, surviving on meagre rations of corn and salt. Yet anyone caught taking food from fields or orchards faces a lethal beating or being locked in an underground cell too small to stand in.

'When I arrived it was like a scene from a horror movie,' said Kang Chol Hwan, sent to Yodok concentration camp aged nine after his grandfather was accused of sedition. 'I once watched a film about Auschwitz and I could relate so much to the situation.' He was always hungry during a decade in the camp. Many inmates die from malnutrition in the first few months. 'We were never given protein so we would catch snakes, rats, even insects.'

Kang spoke of ceaseless fear during his incarceration, with beatings commonplace. Once he had to watch a group of people being hanged, then guards made them throw stones at the dangling dead. 'They left the bodies so crows came and ate the flesh,' he added.

Almost all the inmates are locked up following arbitrary arrest, with many having no idea what 'crime' they or their relatives have committed. Offences have included leaving dust on a picture of the 'supreme leader', holding a religious service and listening to foreign radio.

Jung Gwang Il spent three years in Yodok after being arrested for alleged spying. He was so badly tortured even before entry he could only crawl in on his hands and knees, having lost half his body weight after months held in stress positions. 'It was a living hell. All the people were suffering from malnutrition. They did not look like human beings,' he said. Jung, 54, said any sense of solidarity was crushed from prisoners ordered to inform on fellow inmates or face savage punishment. 'People believe that once you show sympathy towards others, it will bite you back,' he said. 'I saw a kid who looked so weak I suggested he went to a special area for people in a bad situation. He told a guard. I got beaten up so badly with a thick stick I nearly died. The guard asked me who was I to make decisions for others?'

During winter his 400-strong group worked in forests but, all too often, the tired and hungry prisoners moved too slowly to avoid falling trees chopped down for lumber. 'The guards just laughed as people were crushed.'

Since the ground was frozen and work could not stop, corpses were thrown in a big pile. 'Some were not dead. At night when I went to the toilet, I heard their moaning. These injured people froze to death.'

This gruesome heap grew until the ground thawed in spring. 'It was like a huge pile of garbage. They were all rotten,' said Jung, who now smuggles films and human-rights information back into the state that betrayed him so badly. He remains deeply traumatised, despite defecting more than a decade ago.

Noh Hui Chang, one of the most senior defectors this century, said: 'All citizens are aware of the camps. This is a regime of fear, everyone is so scared of what might happen.'

Noh, former head of the ruling party's youth wing, fled in 2014 after his close ally – and Kim Jong Un's uncle – was killed by the new leader. During our discussion, he cried after revealing that his wife, children and brothers were sent to a camp after his escape.

Ahn Myung-Chul, who worked as a guard in four different camps, said: 'Those who die are the lucky ones. This is modern-day slavery, torturing people over decades.'

Satellite evidence suggests some of these barbaric units have grown since Kim Jong Un (pictured) took over the family dictatorship six years ago

For his first three years, brainwashed and believing the propaganda, Ahn beat inmates and used them as targets for martial arts practice. Then one day he was bored and talked to a prisoner, suddenly discovering they were innocent people.

He said some guards enjoyed torture. One day his boss killed an elderly prisoner in a brick factory by smashing a metal bar on his head. 'He was never punished for this.'

Guards could even advance their ambitions though murder. 'One man dragged two men and three women to the fences, then shot them claiming he'd caught them escaping. He was rewarded with admission to a top university.'

In atonement for past misdeeds, Ahn now campaigns against these camps, including the one holding his mother and sister. 'I wish I did not know the details but I know how they will be doing,' he said sadly.

Three years ago the United Nations condemned North Korea's camps as crimes against humanity and without parallel on the planet, yet even now there is no reliable footage from camps the size of cities. Ahn worked at one that is 30 miles long and 25 miles wide.

'Their size is matched only by the cruelty,' said Thor Halvorssen, of the Human Rights Foundation, which assists defectors. 'Not since the concentration camps of the Nazis and gulags of Stalin has humanity seen such a meticulously organised system to punish and dehumanise a population.'

Today the middle-aged Lim lives in Seoul, just 35 miles but a world away from the despotic regime that held her in its rigid grip.

She defected 15 years ago after a short spell in prison, deciding to flee when forced to parade naked before male guards.

She fears the camps have become even worse since she left and is angry at the system she served once with such loyalty.

'I feel so betrayed by leaders who lied to us,' she said. 'We were told not to see these people as humans. Now I feel traumatised.'