(UPDATE) SEOUL: North Korean leader Kim Jong Un oversaw the test firing of a "new-type surface-to-sea missile," state media reported on Thursday, part of a purported fresh drive to ramp up maritime border security.

So far this year, Kim has declared South Korea his country's "principal enemy," jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 millimeter" of territorial infringement.

Pyongyang has also doubled down on weapons tests, including launching a string of cruise missiles, testing an "underwater nuclear weapon system," and firing a solid-fueled hypersonic ballistic missile.

Seoul's military said on Wednesday it had detected the launch of multiple cruise missiles by the North.

The official Korean Central News Agency (KCNA) said Kim had overseen the "evaluation test-fire of new-type surface-to-sea missile Padasuri-6 to be equipped by the navy."

He expressed "great satisfaction over the results of the test-fire," it added.

The missiles hit their intended targets after flying over the East Sea for 1,400 seconds, it said.

Kim said South Korea let "various kinds of battleships intrude into the waters of the DPRK to seriously encroach upon its sovereignty," referring to the North by the acronym of its official name, the Democratic People's Republic of Korea.

The maritime border between the two countries, known as the Northern Limit Line, has never been officially demarcated, and has been the site of previous clashes between the two Koreas.

Kim decried Seoul's "desperate efforts to preserve 'northern limit line,'" and vowed that the North would "thoroughly defend the maritime sovereignty by force of arms and actions, not by any rhetoric."

Earlier this year, Kim said he would no longer recognize the Northern Limit Line, and according to Thursday's KCNA report, he referred to the de facto border as a "ghost one without any ground in the light of international law."

Earlier this week, North Korea announced it had tested a new control system for a multiple rocket launcher it said would have an "increased" battlefield role.

The cruise missile launches have prompted speculation from experts that the North is testing the weapons before shipping them to Moscow for use in Ukraine.