North Korea has vowed military action if South Korea fires inside its territorial waters.
The North Korean military statement is a response to an incident on Friday in which South Korea said its navy fired warning shots at North Korean fishing boats that crossed a disputed Yellow Sea boundary.
North Korea says Seoul's navy is using another unnamed nation's fishing boats to frame North Korean boats and to trespass.
It vows to "wage a great war" if there is more firing.
Fishing boats from China and both Koreas routinely jostle for position during Yellow Sea crab-catching season.
Several naval clashes since 1999 have left a few dozen dead.
Pyongyang does not recognise the maritime boundary that was drawn by the US-led UN Command at the end of the 1950-1953 Korean War.