North Korea threatened Friday to attack South Korean warships without warning if they continue to violate the western sea border.

North Korea claimed that South Korean Navy speedboats made a "military provocation" by deeply intruding into the North's territorial waters two or three times a day between May 1 and 7, according to the Korean Central News Agency.

In what it called an "emergency special warning," the North said the South's military claimed they were clamping down on illegal fishing by a third country, but it was really a secret operation to cross the border in the tension-high Yellow Sea.

"From this moment, it will make a sighting strike without any prior warning at any warship of the South Korean Navy intruding into the extension of demarcation line in the hotspot (of the sea)," the KCNA said.

Pyongyang does not acknowledge the border, known as the Northern Limit Line, which was drawn unilaterally by the U.S.-led United Nations Command when the 1950-53 Korean War ended in a ceasefire. The North has long demanded that the line be drawn farther south. The two Koreas fought bloody battles in 1999, 2002 and 2009 near the border.

North Korea also threatened to successively deal stronger blows to the South's ships if South Korea makes counterattacks on the North, the KCNA said.

Flatly rejecting the North's claims, South Korea expressed "serious regrets" over the warning against "our ships' normal operations." "It is not our side but your side that brings up tension along the NLL. Your threatening words and deeds by distorting facts are stoking unnecessary military tension between the two Koreas," Defence Ministry spokesman Kim Min-seok said in a message to the North Korean command.

"If you act provocatively while ignoring our warnings, we will sternly and strongly respond to them to the degree where you will bitterly repent," Kim said, calling for Pyongyang's full respect for the maritime border.