SEOUL, South Korea " South Korean and U.S. troops raised their alert Thursday to the highest level since 2006 after North Korea renounced its peace treaty with the allied forces and threatened to strike any ships trying to intercept its vessels.
The move was a sign of heightened tensions on the peninsula following the North's underground nuclear test and its firing of a series of short-range missiles earlier this week.
In response, Seoul decided to join more than 90 nations that have agreed to stop and inspect vessels suspected of transporting banned weapons.
North Korea claims the South's participation in the U.S.-led Proliferation Security Initiative is a prelude to a naval blockade and raises the prospect of naval skirmish in its western waters.
On Wednesday, it renounced the 1953 truce that halted fighting in the Korean War. It said Thursday through its official media that it was preparing for an American-led attack. The U.S. has repeatedly denied it is planning military action.
"The northward invasion scheme by the U.S. and the South Korean puppet regime has exceeded the alarming level," the North's main Rodong Sinmun newspaper said in a commentary, carried by the official Korean Central News Agency. "A minor accidental skirmish can lead to a nuclear war."
South Korean Defense Ministry spokesman Won Tae-jae said the South Korea-U.S. combined forces command has raised its surveillance from a level 3 to a level 2. He said the last time the alert level was that high was the in 2006, when the North conducted its first nuclear test.
A South Korean Joint Chiefs of Staff officer, speaking on condition of anonymity citing department policy, said the South's military has also bolstered "personnel and equipment deployment" along its land and sea borders.
He said, however, that there has been no particular movement of North Korean troops along the heavily fortified border areas.
There are 28,500 U.S. troops in South Korea and another 50,000 in Japan. All are within striking range of North Korea's missiles.
Though the officer refused to give details, South Korea has recently deployed more anti-air missiles and artillery at its military bases on islands near the disputed western sea border with North Korea, South Korea's mass-circulation JoongAng Ilbo newspaper reported Thursday.
A South Korean destroyer has also been deployed near the sea border to prepare for any provocations, the paper said.
Seoul has said its military was prepared to "respond sternly" to any North Korean provocation, and would be able to contain the North with the help of U.S. troops.
The U.N. Command on Korea issued a statement defending the armistice, and said it would continue to observe it.
"The armistice has served as the legal basis for the cease-fire in Korea for over 55 years and significantly contributes to stability in the region," it said. "The armistice remains in force and is binding on all signatories, including North Korea."
Experts said the recent flurry of belligerence from North Korea may reflect an effort by leader Kim Jong Il, who is reportedly grooming one of his sons as his successor, to boost his standing among his impoverished people by generating fear and claiming to be strong in the face of outside threats.
It was also seen testing the new administration of President Barack Obama.
North Korea has announced it was abandoning the armistice several times before " most recently in 2003 and 2006.
The truce doesn't cover the waters off the west coast, and North Korea has used the maritime border dispute to provoke two deadly naval skirmishes " in 1999 and 2002.
Diplomats, meanwhile, discussed further what measures should be taken to punish the North.
U.S. Secretary of State Hillary Rodham Clinton warned of "consequences" but it remained unclear what action the United Nations Security Council would take.
The five permanent veto-wielding council members " the United States, Russia, China, Britain and France " and the two countries most closely affected by the nuclear test, Japan and South Korea, discussed possible U.N. sanctions and other measures for a new Security Council resolution on Tuesday.
A diplomat, who is familiar with the talks but spoke on condition of anonymity because they were closed, said Wednesday there was a clear commitment to go for sanctions in the new resolution and no reluctance from North Korea's allies, China and Russia.
Japanese Prime Minister Taro Aso said a new resolution should be stronger than the one issued after the North's first atomic test in October 2006 and contain sanctions.
Aso also indicated a possibility that Japan may impose own sanctions, though he said he has to see the content of a U.N. resolution currently being discussed.