North Korea has entered "a state of war" with the South, in the reclusive communist state's latest belligerent threat.
The statement came a day after its young leader threatened the United States because two American B-2 bombers flew a training mission in South Korea.
Analysts say a full-scale conflict is extremely unlikely and North Korea's threats are instead aimed at drawing Washington into talks that could result in aid and boosting leader Kim Jong Un's image at home.
But the harsh rhetoric from North Korea and rising animosity from the rivals that have followed United Nations sanctions over Pyongyang's February 12 nuclear test have raised worries of a misjudgment leading to a clash.
In a joint statement by the government, political parties and organisations, North Korea said it would deal with all matters involving South Korea according to "wartime regulations". It also warned it would retaliate against any provocations by the United States and South Korea without "any prior notice".
The divided Korean Peninsula is already in a technical state of war because the 1950-53 Korean War ended in a ceasefire, not a peace treaty. But Pyongyang said it was scrapping the war armistice earlier this month.
South Korea's Unification Ministry said the latest threat was not new and just a follow-up to Kim's earlier order to put troops on a high alert in response to annual US-South Korean military drills. Pyongyang sees those drills as rehearsals for an invasion; the allies call them routine and defensive.
Mr Kim warned his forces were ready "to settle accounts with the US" after two nuclear-capable US B-2 bombers dropped dummy munitions on a South Korean island range as part of joint drills and returned to its base in Missouri. North Korean state media later released a photo of Kim and his senior generals huddled in front of a map showing routes for envisioned strikes against cities on both American coasts. The map bore the title "US Mainland Strike Plan".
At the main square in Pyongyang, tens of thousands of North Koreans turned out for a 90-minute mass rally in support of Kim's call to arms.
Meanwhile, North Korea has threatened to shut down a factory complex which is the last major symbol of inter-Korean co-operation. A spokesman for the North's office controlling the Kaesong industrial complex said that it would close the factory park just across the border in North Korea if South Korea continued to undermine its dignity.