North Korea says it has put its military on full combat alert in advance of a big military exercise by United States and South Korean forces.
The official news agency called the manoeuvres "a dangerous provocation".
North Korea has warned that any attempt to shoot down a satellite it says it plans to launch will result in war.
The South and the US believe North Korea could be preparing to test-fire a long-range missile under the guise of a satellite launch.
On Friday, the North said that the risk of conflict meant it could no longer guarantee the safety of commercial flights through airspace it controls off the east coast.
A number of airlines have already re-routed their flights as a precaution.
In a statement published by the official Korean Central News Agency on Monday, the Korean People's Army warned that it was ready to use force against the South, the US and Japan.
"We will retaliate any act of intercepting our satellite for peaceful purposes with prompt counter strikes by the most powerful military means," it said. "Shooting our satellite for peaceful purposes will precisely mean a war."
The army earlier issued a separate statement saying all military personnel had been ordered "to be fully combat ready" in order to defend the nation.
It described the joint military exercise as "unprecedented in the number of the aggressor forces involved and in their duration".
The army added that it would cut off North-South military communications during the exercise period.
The 12-day military exercise involves about 50,000 US and South Korean troops, in what the two allies say is a rehearsal for the defence of the peninsula, the BBC's reports.
But North Korea is always rankled by the annual drill, claiming that it is a dry run for an invasion and dangerously provocative.