North Korea says it has abandoned the truce that ended the Korean war, amid rising tension in the region.

It blamed its decision on South Korea joining a US-led initiative to search ships for nuclear weapons.

It said the South's actions were a "declaration of war", and pledged to attack if its ships were stopped.
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Meanwhile, South Korean news reports say that steam has been seen coming from a plant at the North's main nuclear facility, a sign that it has made good on its threat to restart efforts to make weapons-grade plutonium.
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The immediate cause of North Korea's actions, it said, was South Korea's announcement on Tuesday that it would definitely join the Proliferation Security Initiative (PSI) - a US-led campaign to search ships carrying suspicious cargoes to prevent trafficking of weapons of mass destruction.

Joining the PSI "is a natural obligation", South Korean Foreign Minister Yu Myung-hwan told reporters. "It will help control North Korea's development of dangerous material."

But North Korea's response has been unequivocal.

"Any hostile act against our peaceful vessels, including search and seizure, will be considered an unpardonable infringement on our sovereignty," a spokesman for the North's army told KCNA.

"We will immediately respond with a powerful military strike."

On Wednesday Pyongyang renounced the 1953 armistice and the following day warned U.S. forces against advancing into its territory.

"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."

The Army’s top military officer on Thursday said that his service would be well-prepared to fight in a conventional war against North Korea should it ever get to that.

“We have a combat seasoned force,” said Gen. George Casey, the Army’s chief of staff, in a discussion hosted by the Center for Strategic and International Studies.
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“It’s a combat seasoned army that can change gears, but it is not as fast we would like,” he said, adding that the Air Force and Navy, already with a good footprint in the region, would be able to stage a quick response.

US Senator John Kerry said China will support UN Security Council action on North Korea and agrees the isolated regime must face the consequences of this week's nuclear test.

"As for the UN Security Council action on North Korea, Foreign Minister Yang (Jiechi) agreed with us that North Korea's actions were wrong and that there need to be consequences," Kerry told journalists in Beijing.