Image caption Activists have been staging protests over the fate of North Korean refugees

A new report has shed fresh light on life in North Korean labour camps.

It presents a detailed picture of torture, forced abortions and public executions in a vast network of secret political camps.

The research backs previous evidence that more than 150,000 political prisoners are being held in North Korea.

The authors are calling for an international commission of inquiry to investigate.

The report - by the Washington-based Committee for Human Rights in North Korea - uses detailed satellite photographs to identify barracks, work sites and execution grounds in forced labour camps hidden in remote mountain areas.

"An entire system of political repression in North Korea needs to be eliminated," said the committee's chairwoman, Roberta Cohen.

The researchers relied for their information on some 60 former prisoners and camp guards, who have escaped to South Korea.

They describe a complex web of prisons and work camps built to punish those seen as resistant to North Korea's system of total state control, including those who try to escape to South Korea.

They say that many inmates die from malnutrition and harsh working conditions as they labour in mines, factories and on farms.

Jailed 'for decades'

The researchers also document witness accounts of public executions - mainly of prisoners who try to escape or are caught stealing extra food rations.

They also present fresh evidence of routine torture, beatings and forced abortions - mainly for women who are repatriated from China and are thought to have conceived children by Chinese men.

Former inmates say they were sent to the camps - sometimes for decades - without any judicial process or often without hearing the charges against them.

Children and other family members of the accused can also be incarcerated in an effort to eradicate all criticism of North Korea's political system.

One woman said she was imprisoned for singing a South Korean song, and others for having relatives with South Korean or Japanese connections.

The report's author, David Hawk, first documented the extent of the North Korean camp system in his report "Hidden Gulag" in 2003.

He says he was able to update his research because many more North Korean refugees have since escaped from the country with fresh evidence. There are currently about 30,000 refugees in Seoul compared to just a few thousand a decade ago, hundreds of whom have experienced life in the camps.

North Korean officials have told the United Nations in the past that there are no political prisons in the country.