North Korea's military has gone on full alert and threatened "merciless retaliatory blows" just hours before US and South Korean forces began annual military exercises.
Pyongyang has also warned of war if what it calls a satellite launch - but what intelligence analysts believe is cover for a long range missile test - is shot down.
"Shooting our satellite for peaceful purposes will precisely mean a war," said a statement carried on the North's official Korean Central news agency (KCNA) on Monday.
The statement warned of "prompt counter strikes by the most powerful military means".
A day earlier North Korea threatened to "deal merciless retaliatory blows" if US and South Korean forces intrude "even an inch" into North Korean territory.
The US and South Korea began joint military exercises on Monday involving 26,000 US troops, more than 30,000 South Korean soldiers and the aircraft carrier USS Jong C Stennis.
Commanders say the Key Resolve-Foal Eagle exercises are designed to test the ability to deploy troops and equipment to the Korean peninsula in the event of an emergency.
The North has described the 12 days of operations taking place in more than 20 locations across South Korea as "extremely adventurous and dangerous military provocations … an undisguised military threat".
In a statement the Korean People's Army described the exercises as "unprecedented in the number of the aggressor forces involved and in their duration".
"A war will break out if the US imperialists and the warmongers of the South Korean puppet military hurl the huge troops and sophisticated strike means to mount an attack."
A separate statement said that North Korea would cut off military communications lines with the South during the exercises since maintaining normal channels would be "nonsensical".
Pyongyang regularly accuses the US and South Korea of hostile intentions before the annual exercises, which have been held for years without major incident.
On Sunday, KCNA accused the Americans of being "arch criminals prodding the North and the South into armed conflict".
Tensions on the Korean peninsula have been rising, with Pyongyang declaring last month that it was "fully ready for an all-out confrontation" with South Korea.
The North announced in January that it was scrapping all agreements with the South, including a 1991 pact that recognised the Northern Limit Line, a sea border drawn unilaterally by US-led United Nations forces after the 1950-1953 Korean war.
On Thursday, Pyongyang said it could not guarantee the safety of the South's commercial flights off the east coast of the peninsula, where the missile launch site is believed to be located.
Airlines have said they would avoid North Korean airspace as a precaution, while the South Korean government has demanded that Pyongyang withdraw what it called an "inhumane" threat to civilian aircraft.