The North stressed that provocation would include any attempt to interfere with its launch of a satellite into orbit. U.S. and Japanese officials suspect the launch is a cover for a test of a long-range attack missile and have suggested they might move to intercept the rocket.
"Shooting our satellite for peaceful purposes will precisely mean a war," North Korea's military said in a statement carried by the official Korean Central News Agency. Any interception attempt will draw "a just, retaliatory strike," it said.
The North has been on a retreat from reconciliation since President Lee Myung-bak took office in the South. After Lee said the North must continue dismantling its nuclear program if it wants aid, Pyongyang cut ties, suspended joint projects and stepped up its rhetoric.
"The danger of a military conflict is further increasing than ever before on the Korean peninsula because of the saber rattling which involves armed forces huge enough to fight a war," the North's news agency said.
Allied commanders say the exercises are annual drills the two nations have held for years, while the North has been condemning them as a rehearsal for invasion.
Analysts say North Korea's heated words are designed to grab President Barack Obama's attention. With South Korea cutting off aid, the impoverished North is angling for a diplomatic coup of establishing direct ties with the U.S., analysts say.
For weeks, the North has said it is forging ahead with plans to send a communications satellite into space -- a launch that U.S. and Japanese officials say would violate a U.N. Security Council Resolution banning the North from testing ballistic missiles. That decree came after the North test-fired a long-range missile and conducted an underground nuclear weapon test in 2006.
Analysts say the launch could occur late this month or in early April, around the time North Korea's new parliament, elected Sunday, convenes its first session with leader Kim Jong Il at its helm.
Kim, 67, was among legislators unanimously elected to a five-year term, the North's state media said. Elections in North Korea are largely a formality, with the ruling Workers' Party handpicking one candidate for each district and voters endorsing the sole nominee.
Observers were watching the results for signs of a shift in policy -- or hints that Kim, who reportedly suffered a stroke last August, might be grooming a son to succeed him. None of his three sons appeared on a list of lawmakers announced on state TV late Monday.
In Seoul, Obama's special envoy on North Korea, Stephen Bosworth, urged Pyongyang not to fire a missile, which he said would be an "extremely ill-advised" move.
"Whether they describe it as a satellite launch or something else makes no difference," Bosworth said after talks with his South Korean counterpart on drawing Pyongyang back to international talks on the North's nuclear disarmament.
South Korea's Defense Ministry spokesman, Won Tae-jae, played down the North's threats as "rhetoric," but added that the country's military was ready to deal with any contingencies.
Hundreds of South Koreans were stranded in the northern border town of Kaesong after Pyongyang severed the last communications link between the two governments to protest the U.S.-South Korean military exercises that began Monday.
North Korea banned nearly all cross-border traffic in December amid deteriorating relations with Seoul but has allowed a skeleton staff of South Koreans to work at a joint industrial zone in Kaesong that is a crucial source of hard currency for the isolated communist regime.
The two Koreas use the hot line to coordinate the passage of people and goods through the heavily fortified Demilitarized Zone, and its suspension shut down traffic and stranded about 570 South Koreans north of the border.
All South Koreans in Kaesong are safe, Seoul's Unification Ministry said as it called on Pyongyang to restore communications.
Cutting the hot line for the duration of the 12-day U.S.-South Korean maneuvers leaves the two Koreas without any means of quick, direct communication at a time of high tension, when even an accidental skirmish could trigger fighting.
North and South Korea technically remain in a state of war since their 1950-53 conflict ended in a cease-fire, not a peace treaty. Hundreds of thousands of soldiers are massed on each side of the DMZ.
The United States, which has about 28,000 military personnel in South Korea, routinely holds joint military exercises with the South.
Last week, the North threatened danger to South Korean passenger planes flying near its airspace if the maneuvers went ahead, and several airlines rerouted their flights as a precaution.
Gen. Walter Sharp, the U.S. commander, said the joint exercises -- involving some 26,000 U.S. troops, an unspecified number of South Korean soldiers and a U.S. aircraft carrier -- are "not tied in any way to any political or real world event."