South Korea 's Yonhap News Agency said North Korea might be using a vertical cold-launch system to fire the missiles. The system, which is commonly associated with submarine- and ship-based weapons, ejects a missile from its container before the missile ignites midair.

North Korea has test-fired sea-to-surface strategic cruise guided missiles, the North's state media reported Sunday, quoting Kim as saying that Pyongyang's deterrence means are being perfected.

Pyongyang expresses its discontent over the "concerning provocations" that the combined aerial maneuvers carried out last week between Seoul, Tokyo, and Washington represented. North Korea has announced a test with strategic cruise missiles,