US and South Korean forces raised their military alert level today, a day after North Korea renounced the 55-year-old truce on the peninsula and threatened war if its ships are searched for weapons of mass destruction.

Three days after North Korea carried out an underground nuclear test, the US-South Korea combined forces command moved its level of surveillance to the second-highest level on its scale of five, the highest since North Korea’s first nuclear test in 2006.

Meanwhile, the North’s state media accused the allies of plotting an attack, and warned that small incidents could have disastrous consequences. “The northward invasion scheme by the US and the South Korean puppet regime has exceeded the alarming level,” the Workers’ Newspaper said in an editorial. “A minor accidental skirmish can lead to a nuclear war.”

In Seoul, the spokesman for South Korea’s defence ministry said: “We are maintaining a tight defence posture to prevent the North's military provocations. The military will deal sternly with provocative acts.” The South Korean JoongAng Ilbo newspaper reported that the Government has deployed extra anti-air missiles and artillery on five disputed islands in the Yellow Sea near the disputed sea border with the North.

Times Archive, 1950: Communists move into South Korea

The invasion by North Korea is being resisted by the South Korean forces, who have asked General MacArthur for assistance

• Topic: The Korean War

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South Korea has 670,000 soldiers supported by 28,500 South Korean troops and, since the Korean War ended in an Armistice in 1953, they have engaged in only minor exchanges with the million-strong army of the North.

“The armistice has served as the legal basis for the ceasefire in Korea for over 55 years and significantly contributes to stability in the region,” said a statement by the United Nations command, the flag under which the US and its allies fought the war. “The armistice remains in force and is binding on all signatories, including North Korea.”

In Washington, the US Secretary of State, Hillary Clinton, said: “I want to underscore the commitment the United States has, and intends always to honour, for the defence of South Korea and Japan.” But there was no sign of panic, or reports of any preparation for large-scale military mobilisation in either North nor South. “We're certainly concerned and take any threat seriously,” the State Department spokesman Robert Gibbs said. “But my sense is they're trying to get renewed attention through sabre-rattling and bluster and threats.”

The announcement that the North would no longer observe the Armistice after South Korea joined the Proliferation Security Initiative, a multilateral effort to limit the spread of weapons of mass destruction by intercepting shipping and aviation. The North has threatened to abandon the Armistice five times in the past 15 years.

But after the underground nuclear test, then drills involving the firing of short-range missiles, and finally the renunciation of the Armistice, the question is what the North might do next. South Korean media speculated that Pyongyang might test launch another long-range missile, or engineer a skirmish on land or sea.

“The Democratic People’s Republic of Korea [North Korea] has tremendous military muscle and its own method of strike able to conquer any targets in its vicinity at one stroke or hit the US on the raw,” the Korean People's Army said on Wednesday. “Those who provoke the Democratic People's Republic of Korea once will not be able to escape its unimaginable and merciless punishment.”

Meanwhile, a respected US journal, the Bulletin of the Atomic Scientists, suggested that the test explosion on Monday was not, as Russian estimates had suggested, as big as the bombs that destroyed Hiroshima and Nagasaki. An article on the magazine's website by Jeffrey Park, director of the Yale Institute for Biospheric Studies, estimates its size at four kilotonnes based on measurements of the artificial earthquake that it generated.

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