'Declaration of war'
The North's warning came days after Kim Tae-Young, the South's defence minister, reiterated that Seoul would launch a pre-emptive strike if there were indications that the North was preparing a nuclear attack.
"We would have to strike right away if we detected a clear intention to attack [South Korea] with nuclear weapons"
Kim Tae-Young, South Korea's defence minister
"We would have to strike right away if we detected a clear intention to attack [South Korea] with nuclear weapons," Kim told a Seoul forum on Wednesday.
"It would be too late and the damage would be too big if, in the case of a North Korean nuclear attack, we had to cope with the attack."
Kim made similar remarks in 2008 when he was chairman of the South's joint chiefs of staff.
North Korea also reacted angrily at the time, temporarily expelling South Korean officials from a Seoul-funded industrial park at Kaesong just north of the heavily fortified border.
Tensions between the two nations rose after Pyongyang pulled out of six-party talks on its nuclear programme last year following widespread condemnation of a second long-range missile launch following its first in 2006.
International efforts to bring North Korea back to six-party talks have so far been unsuccessful.
Its foreign ministry repeated last week that it would not return to the talks with the United States, China, South Korea, Russia and Japan until United Nations sanctions are lifted.
The United States and South Korea have rejected the demands, saying the North must first return to the disarmament talks.
Kurt Campbell, the US assistant secretary of state, will visit Japan and South Korea early next month to discuss regional security issues including ways to revive the six-party talks.