"From this time on, there will be unannounced targeted strikes against puppet navy vessels that violate our military's maritime border in the west coast hotspot waters," the North Korean news agency KCNA quoted the country's military command statement as saying on Friday.
In case of any armed response, the Korean People's Army (KPA) pledged to "successively deal strong second, third and more retaliatory strikes," according to the statement.
The KPA claimed that South Korean naval speedboats made a "military provocation" by crossing into the North's territorial waters in the Yellow Sea in the first week of May "under the pretext" of intercepting Chinese fishing boats.
The KPA said North Korea's military would "make a sighting strike without any prior warning" on any South Korean naval ship thought to have entered its territorial waters.
South Korea's Office of the Joint Chiefs of Staff for its part, described Pyongyang's as "insane", saying that its naval vessels have not crossed the maritime border since the end of the 1950-1953 Korean War.
The western maritime border has been the scene of deadly naval clashes between the two Koreas in the past.
In 2010, at least 46 South Korean sailors were killed after a torpedo hit their navy ship near the two's disputed maritime border. Seoul points the finger at North Korea, which vehemently denies the accusations.