The US, South Korea and Japan held a joint naval missile defence exercise on Sunday to counter North Korea's evolving nuclear and missile threats, the South's navy said, days after the North launched an intercontinental ballistic missile (ICBM). North Korea fired its latest Hwasong-18 missile, which Pyongyang describes as the core of its nuclear strike force, off the east coast on Wednesday in what it said was a "strong practical warning" to the adversaries.