SEOUL( Reuters)-North Korea fired at least one ballistic missile towards the sea off its east coast, the South Korean military said on Sunday, a day after a top U.S. envoy departed Seoul. Seoul's Joint Chiefs of Staff said North Korea had fired an" unknown" missile, without elaborating.In recent weeks, North Korea has test fired a range of missiles, including its largest intercontinental ballistic missile( ICBM).U.S. Special Representative Sung Kim met his South Korean and Japanese counterparts,