WASHINGTON (AFP) - North Korea has increased economic linkages with a prison camp where inmates are working by hand in agriculture, according to satellite imagery released on Thursday by a human rights group.

The Committee for Human Rights in North Korea, a Washington-based group that is trying to shed light on the secretive state's record, said that Camp Number 25 near the north-eastern city of Chongjin had about 79 ha of cultivated farmland including fields for grain crops, orchards and greenhouses.

Satellite imagery from the company AllSource Analysis shows three bridges that connect the camp to a neighbouring village as well as new work over the past year that includes the construction of retaining walls and the grading of land along the banks of a stream.

Mr Greg Scarlatoiu, executive director of the committee, said that prisoners were known to have been "subjected to torture, induced malnutrition and forced labor," but that it has proven difficult to assess the exact economic benefits.

The satellite imagery "suggests that the economic significance of the Camp 25 prison labour has been increasing over the past 12 months," he said.

There were no signs of mechanical equipment, indicating that all prisoners work by hand. The report said it was impossible to determine the exact number of prisoners, but previous reports from defectors estimate there are about 5,000.

UN inquiry commission estimated in February that North Korea kept 80,000 to 120,000 political prisoners in four large camps including Number 25, where deliberate starvation was used for control and punishment, and that abuses were also rife in ordinary prisons.

The head of the inquiry, Australian judge Michael Kirby, likened human rights abuses to those of Nazi Germany and urged greater international action.