SEOUL, Feb 19 (Reuters) - The United States held joint air exercises bilaterally with South Korea and Japan involving strategic bombers on Sunday, a day after North Korea fired a Hwasong-15 intercontinental ballistic missile (ICBM) in a "sudden launching drill". 1, came after Pyongyang threatened on Friday an "unprecedentedly persistent, strong" response as South Korea and the United States geared up for their annual military exercises as part of efforts to fend off the growing nuclear and missile threat that the North poses. "Analysts say North Korea is likely to conduct more weapons tests, including a possible new solid-fuel missile which could help the North deploy its missiles faster in the event of a war