North Korea announced that it is to abandon the armistice which for more than 50 years has held together a fragile peace with South Korea, amid increasing military activity on the peninsula.

The news came as reports emerged in the South Korean media that North Korea has restarted its plutonium reactor, which produces fuel for nuclear warheads such as the one it tested on Monday.

US spy satellites have detected steam coming out of the nuclear power plant at Yongbyon since the end of April, the Seoul-based Chosun Ilbo newspaper reported.

The doors of storage facilities at the site, north of Pyongyang, have been opened and closed a number of times and vehicles have been detected apparently transporting chemicals.

The activity suggests that North Korea is reprocessing spent plutonium fuel rods to produce the fissile material used to build Pyongyang’s nuclear arsenal.

If all 8,000 fuel rods are processed, the newspaper said, they would yield between 6kg and 8kg of plutonium, on top of the 31kg that North Korea admitted having stockpiled last year.

This could add one nuclear weapon to the six to 12 bombs that North Korea is already believed to have assembled.

The North has threatened to abandon the armistice before, so the announcement does not mean that war is imminent. But it could suggest that Pyongyang is preparing for a military skirmish with the South, as the next move in its strategy of gradually increasing pressure on the international community.

The move comes in response to South Korea’s announcement two days ago that it is to join the Proliferation Security Initiative, a multilateral effort to limit the spread of weapons of mass destruction by intercepting shipping and aviation.

"Our military will no longer be bound by the armistice accord as the current US leadership ... has drawn the puppets [South Korea] into the PSI,” Pyongyang said in a statement.

Signed in August July 1953, after months of negotiations, the armistice marked a ceasefire and truce rather than a formal end to the Korean War, and the past 50 years have been marked by bitter arguments about what it should be replaced with.

North Korea wants a peace treaty with the United States, but the US insists that any settlement must include South Korea.

"The Korean peninsula will go back to a state of war [without the armistice],” said the statement, which was issued through the Korean Central News Agency.

"Those who have provoked us will face unimaginable merciless punishment … Any hostile act against our peaceful vessels, including search and seizure, will be considered an unpardonable infringement on our sovereignty and we will immediately respond with a powerful military strike.”

According to the South Korean government, North Korean fighter jets have more than doubled their sorties near the demilitarised zone, which divides the two states.

North Korea test-fired another short-range missile overnight, bringing to five the number it has fired since the underground nuclear test on Monday morning, the South Korean news agency, Yonhap, said.

Unlike the long-range rocket that was fired deep into the Pacific early last month, the testing of such smaller weapons does not violate international law or UN resolutions.

However, it is a further provocation to North Korea’s international critics — as well as a reminder of the country’s considerable conventional defensive capabilities to any government that might be contemplating military action.

The US sought to form a united front against North Korea yesterday in response to the latest missiles launches and the underground nuclear test on Monday which provoked outrage across the world.

It remains to be seen whether the unusual consensus on the UN Security Council will extend to concrete steps such as sanctions.

After an emergency meeting on Monday, the Security Council issued a unanimous statement condemning Pyongyang for its nuclear test, and representatives of its five veto-bearing members as well as Japan and South Korea discussed a new resolution last night.

Before that meeting Susan Rice, the US Ambassador at the United Nations, vowed that Pyongyang would pay the price for its actions.

Hillary Clinton, the US Secretary of State, telephoned Sergei Lavrov, the Russian Foreign Minister, to ask for a unified response.

Ian Kelly, the State Department spokesman, said: "We look forward to working with our colleagues on the council to craft strong, unequivocal and unified response to North Korea’s grave violation of international law.”

Mr Kelly carefully avoided mentioning sanctions, a recognition that Russia and China are unlikely to back such a move.

Moscow has condemned the nuclear test and made clear that it would support firm United Nations action, but ruled out further curbs on North Korea.

"Most likely the adoption of a tough UN Security Council resolution is unavoidable,” a Russian Foreign Ministry source said.

"The reaction should be fairly serious because the authority of the Security Council is at stake.”

China, which is North Korea’s closest ally, voiced its "resolute opposition” to the nuclear test in a rare instance of open criticism of its communist neighbour. However, Beijing would not be prepared to tighten sanctions already imposed by the UN after its previous nuclear test.

The Times