The chairman of the Joint Chiefs of Staff called for increased naval combat readiness on Monday during his inspection of the Navy's combat commands, the JCS said.

Gen. Lee Sun-jin's visits to the Navy's Fleet and Submarine Force Commands and the underwater demolition team came as North Korea has ratcheted up its belligerent anti-South rhetoric to an unusually high level.

Over the last month, North Korea has threatened to launch nuclear attacks on South Korea and strike South Korea's presidential office Cheong Wa Dae and other government institutions.

It has also claimed to have made progress on its development of nuclear-mounted intercontinental ballistic missiles and solid rocket fuel, having conducted its fourth nuclear test and long-range rocket launch this year.

Earlier this month, North Korea had sent jamming signals in a provocative weeklong operation to disrupt South Korea's GPS.

"North Korea is stepping up its naval threats day by day, having conducted large-scale landing and counter-landing exercises in March and continuing its development of submarine-launched ballistic missiles," the JCS chairman said during the visit.

Lee called on naval service members to "make every effort to enhance their surveillance over the North and maintain a swift response posture in order to be able to detect and deal with any enemy vessels or submarines," according to the JCS.

"If the enemy launches a reckless provocation, (our forces) should hit back resolutely and in a proportional manner," he said.

Aboard the Navy's 214-Type submarine, Lee also checked the vessel's precision attack capability and armament, the JCS said. (Yonhap)