A female prison guard has spoken of savage beatings and killings at a North Korean camp. Lim Hye-jin says she was so traumatised that she could not eat for days afterwards.

One of the worst episodes was the killing of seven family members of two brothers who managed to escape. Many more prisoners were beaten as revenge for the prison breakout. After the two brothers were recaptured, they were "beheaded in front of everyone," Lim said.

"They called everyone to watch as a warning not to flee. The other prisoners then had to throw stones at them."

The former prison guard also recounted horrific scenes of routine killings and the rape of political prisoners who had been declared enemies of the state.

"They do not see them as human beings, just as animals," Lim said.

She spoke to the Mail On Sunday about her experiences of how brutally prisoners were treated. Lim recounted one incident where a woman was stripped naked, then set on fire after being interrogated by a guard.

Lim, a former North Korean prison guard ended up incarcerated herself after she was discovered in illegally trading with China. She is the first female prison guard to speak publicly 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."

In Kwanliso 15 and Kwanliso 25, two of the largest camps, most inmates have committed no crimes but are being punished because they are related to others believed threatening by the regime, according to Amnesty International.

There are four known political prison camps, and over 20 prison labour camps throughout North Korea. Kim Il-sung, North Korea's first leader, drew on the Soviet gulags as the inspiration for the prison camps. Over the past 60 years, the prison system greatly increased, reports the Committee for Human Rights in North Korea.

Today it is estimated that between 80,000 and 120,000 people are imprisoned in these camps. Some prisoners become deformed due to back-breaking work in deep mines, with many dying in accidents. Food is so scarce that inmates catch snakes in order to avoid starvation.

Ahn Myung-Chul, another ex-prison guard who worked in several different camps, said: "Those who die are the lucky ones. This is modern-day slavery, torturing people over decades."