New satellite pictures from North Korea show one of the country's most infamous prison camps has been significantly expanded - and a large civilian population now lies within its boundaries.

As the UN prepares to vote on new sanctions against the reclusive state, Amnesty International has published pictures showing that Pyongyang's authorities have built fences and guard towers all around the Ch'oma-Bong valley, home to the infamous prison camp 14.

The new perimeter extends way beyond the old boundaries. In fact, over the past seven years the government has sealed off virtually the entire valley, erecting 20km of perimeter fencing.

Activists now fear thousands of civilians are trapped within the security zone, subject to severe restrictions.

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

"The creation of a security perimeter with controlled access points and guard towers beyond what appears to be the formal boundaries of Camp 14 blurs the line between more than 100,000 people who suffer in North Korea's Kwan-li-so system and the neighbouring civilian population."

"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," said Rajiv Narayan, North Korea Researcher for Amnesty International.

The Ch'oma-Bong valley is located 70km northeast of Pyongyang. The exact population of the area cannot be verified.

Amnesty has called for the UN to establish an independent commission of inquiry to "investigate the grave and systematic human rights abuses that continue under North Korean leader Kim Jong-un's rule."

Nuclear threat

Meanwhile Pyongyang has threatened to launch a pre-emptive nuclear attack against the US in retaliation to new sanctions targeting North Korean diplomats, cash transfers and access to luxury goods, due to be approved by the UN Security Council.

"Since the United States is about to ignite a nuclear war, we will be exercising our right to a pre-emptive nuclear attack against the headquarters of the aggressor in order to protect our supreme interest," a spokesman for Pyongyang's Foreign Ministry told the state's official KCNA news agency.

"[North Korea] strongly warns the UN Security Council not to make another big blunder like the one in the past when it earned the inveterate grudge of the Korean nation by acting as a war servant for the US in 1950."

The UN resolution has been drafted by the US and China - Pyongyang's closest ally - following its recent nuclear test.

North Korea had earlier warned the West it will cancel the 1953 armistice pact that ended the Korean War.