South Korea and the United States tomorrow start the annual Ulchi Freedom Guardian drill aimed at testing combat readiness for a North Korean invasion.
Although largely played out on computers, it involves tens of thousands of South Korean and US troops.
In a statement published today by the official Korean Central News Agency, the North's military accused Washington and Seoul of planning a "dangerous" rehearsal for nuclear war.
"We declare once again that we will mercilessly open the strongest ... pre-emptive strike of our own style any time at our discretion," it said.
North Korean soldiers were ready to "turn the strongholds of aggression into a sea of fire and ashes", the statement said.
The exercise comes as tensions run high on the Korean peninsula following an extended series of North Korean missile and rocket tests in recent months.
UN resolutions bar North Korea from any launches using ballistic missile technology.
But the nuclear-armed communist country has defended the tests as a legitimate exercise in self-defence.
Seoul had proposed holding high-level talks with Pyongyang to discuss family reunions for those separated by the 1950-53 Korean War and other issues of "mutual interest".
But there has been no official response from Pyongyang.
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