North Korea threatened South Korea on Friday, saying it will pay a "dear" price if its Navy speedboats continue to violate the western sea border.

North Korea claimed that the South's military made a reckless military provocation repeatedly in September as its patrol boats intruded into the North's territorial waters, according to the Korean Central News Agency.

It said that on Tuesday alone, a boat belonging to the South Korean Navy's 2nd fleet went more than 4 kilometers across the sea border, and other vessels made five similar provocations afterward.

North Korea insisted that in the wake of the latest Seoul-Washington military drills, the South conducted "ceaseless infiltration" into the northern side of the sea demarcation line.

"The puppet military gangsters of South Korea should not forget even for a moment that they will have to pay a dear price for their extremely adventurous military provocations," the KCNA said.

Pyongyang does not acknowledge the inter-Korean maritime 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' naval forces fought bloody battles in 1999, 2002 and 2009 near the border.

This year North Korea has increased its ballistic missile tests, including the launching of a submarine-launched ballistic missile in late August in defiance of international condemnation.

On Thursday, Pyongyang said it will take "multi-phased eventful measures" to double its capabilities for nuclear attacks against Washington if the U.S. sticks to its hostile policy toward the North. (Yonhap)