North Korea fired several cruise missiles off its west coast early Saturday, the South Korean military said. An unspecified number of cruise missiles were launched around 4 a.m. (7 p.m. GMT) towards the Yellow Sea, the Joint Chiefs of Staff (JCS) said in a statement on Saturday, adding that the specifications of the missiles were in evaluation course. “We have strengthened surveillance and monitoring and are maintaining maximum readiness in close coordination with the United States,” the JCS said.

The firings come three days after Pyongyang launched two short-range ballistic missiles as part of a “simulated tactical nuclear strike” in response to annual joint military exercises, Ulchi Freedom Shield, between the United States and South Korea. South, which still exasperate the reclusive country. Pyongyang sees these drills as a rehearsal for invading its territory, while the two allies claim they are defensive in nature.

On Tuesday, North Korean leader Kim Jong-Un visited a training command post where he detailed his future war plans, including “very intense simultaneous strikes” on key military posts in the south. North Korea has conducted a record number of weapons tests this year and last week made its second attempt to put a spy satellite into orbit, which ended in failure.

In response, Seoul and Washington have stepped up their defense cooperation, staging joint military maneuvers involving advanced stealth aircraft and US strategic assets.

Relations between the two Koreas are at their lowest in years and diplomacy has stalled after failed attempts to discuss Pyongyang’s denuclearization. Kim Jong-Un has declared North Korea an “irreversible” nuclear power and called for increased production of weapons, including tactical nuclear weapons.