The launches come three days after the North launched a pair of short-range ballistic missiles as part of a "tactical nuclear strike drill" prompted by the annual US-South Korean Ulchi Freedom Shield military exercises, which always infuriate the reclusive regime.

Pyongyang views such the drills as a rehearsal for invasion while the two allies say they are defensive in nature.

An unspecified number of cruise missiles were launched at around 4 am (1900 GMT) towards the Yellow Sea, the Joint Chiefs of Staff said Saturday in the statement, adding the specifications of the missiles were being evaluated.

"We have stepped up surveillance and monitoring and are maintaining utmost readiness in close coordination with the United States," the JCS said.

On Tuesday, North Korean leader Kim Jong Un visited a training command post where he detailed future war plans, including "making simultaneous super-intense strikes" at core military posts in the South.

North Korea has conducted a record number of weapons tests this year, and last week carried out its second failed attempt to put a spy satellite into orbit.

Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets.

Relations between the two Koreas are at their lowest point in years, and diplomacy is stalled after failed attempts to discuss Pyongyang's denuclearisation.

Kim has declared North Korea an "irreversible" nuclear power and called for ramped-up arms production, including of tactical nuclear weapons.