North Korea on Sunday accused the South of declaring war by warning earlier this month that it would launch a preemptive strike if it thought its impoverished neighbour was preparing a nuclear attack.
The angry retort from Pyongyang is the latest in what have become increasingly brittle relations between the two Koreas just as the international community tries to lure the North back to nuclear disarmament talks.
South Korea's Defence Minister Kim Tae-young said last week that Seoul would have no choice but to strike first if there were clear signs of a planned nuclear attack by the North.
"Our revolutionary armed forces will regard the scenario for 'preemptive strike' which the south Korean puppet authorities adopted as a 'state policy' as an open declaration of war," its state KCNA news agency quoted a spokesman for the armed forces general staff as saying.
North Korea has twice tested a nuclear device but there are doubts whether it already has the ability to create an atomic weapon. Military analysts say even if it did it probably does not have the technology to build a nuclear warhead small enough to sit on top of a missile.
The two Koreas are still technically at war with the North maintaining an about one million-strong military and backed by an array of artillery which could bring massive damage to the South Korean capital Seoul, barely 50 miles (70 km) from the border.
The smaller South Korea army is backed by around 28,000 US troops on its soil and the US nuclear umbrella in the region.
"(The North's armed forces) will take prompt and decisive military actions against any attempt of the south Korean puppet authorities to violate the dignity and sovereignty of the DPRK and blow up the major targets including the commanding center," KCNA quoted the statement as saying.
"Those seeking to realize their daydream will not be able to escape an unimaginably miserable fate."
Relations between the two have plunged since conservative Lee Myung-bak became president two years ago, demanding an end to a decade of generous aid unless the North made moves to row back on its nuclear weapons programme.
Late last week, the South said it hoped to see the dormant international weapons negotiations resume next month between the two Koreas, China, Japan, Russia and the United States.
The North has said it is willing to end its year-long boycott but has demanded first an end to UN sanctions that have further damaged its crippled economy and direct talks with the United States on a peace deal to formally end the 1950-53 Korean War.
Washington insists that the reclusive state first drop its nuclear weapons ambitions.