North Korea has built a huge "security perimeter" around a camp for political prisoners, restricting movement in nearby villages as part of its "general repression" of its people, according to Amnesty International, the London-based rights watchdog.

The reclusive country's network of political prison camps is believed to hold at least 200,000 people and has been the scene of rapes, torture, executions and slave labour, Navi Pillay, UN High Commissioner for Human Rights, said in January.

Analysis of new satellite images of the area near Camp No 14 in Kaechon shows that the government is "blurring the lines" between its camps and surrounding civilians, Amnesty International said on Thursday.

The images show that between 2006 and 2013, North Korea has constructed 20km of posts around the Choma-Bong valley and its inhabitants, along with checkpoints and guard towers, the group said in a statement voicing fears about government intentions for the valley, 70km north of the capital Pyongyang.

"The security and control adjacent to Camp 14 shows the degree to which general repression and restrictions on the right to liberty of movement have become commonplace in North Korea," Rajiv Narayan, North Korea researcher for Amnesty International, said.

UN inquiry urged

The London-based group called on the UN Human Rights Council, holding its main annual session in Geneva, Switzerland, to launch an international commission of inquiry into grave and systematic violations "including crimes against humanity".

North Korea denies the existence of a network of camps.

A similar UN investigation similar to the one Amnesty International is demanding, made up of independent experts, has said it is documenting war crimes committed by both sides in Syria's conflict so as to build a case for future prosecution.

Pillay, who met two North Korean camp survivors in December, has called for an independent investigation into "one of the worst - but least understood and reported - human rights situations".

Concerns about abuses in the impoverished country have persisted for years, but have been largely overshadowed in international forums by fears over North Korea's attempts to become a nuclear weapons power.

The UN Security Council is expected to vote later on Thursday on a draft resolution in response to North Korea's third underground nuclear test last month.

Amnesty International commissioned DigitalGlobe to take the images and help with their analysis following reports of the possible construction of a new political prison camp adjacent to Camp No 14 in Kaechon, South Pyongyan province.

"We expected to find a new or expanded prison camp. What we found is in some ways even more worrisome," Frank Jannuzi, deputy executive director of Amnesty International USA, said.

The security perimeter beyond what appears to be the formal boundaries of Camp 14 blurs the line between people in the prison system known as Kwan-li-so and civilians, he said.

'Forced hard labour'

New buildings that appear to house workers are probably linked to expanded mining activity in the region, Amnesty International said.

"Many of those held in political prison camps have not committed any crime, but are related to those deemed unfriendly to the regime and detained as a form of collective punishment," it said.

The prisoners face human rights violations including "forced hard labour, denying food as punishment, torture and other cruel, inhuman, or degrading treatment".

Japan and the US called last week for the Security Council to launch an inquiry into alleged violations including torture and execution of political prisoners in North Korea.

The UN body already has an independent investigator on North Korea, Marzuki Darusman, who has said a wider inquiry should examine personal and institutional accountability for crimes.

Activists hope the forum adopts a resolution on North Korea by consensus now that neither China nor Russia are members.