SEOUL—North Korea fired an “unknown projectile” on Wednesday which appeared to fail immediately after launch, South Korea’s military said, amid reports that the nuclear-armed North is seeking to test-fire its largest missile yet. The United States and South Korea have warned that North Korea may be preparing to launch an intercontinental ballistic missile (ICBM) at full range for the first time since 2017. The projectile was fired from Sunan Airport outside the North Korean capital of Pyongyang, South Korea’s Joint Chiefs of Staff (JCS) said in a statement. “It is presumed that it failed immediately after launch,” the statement said. The projectile seemed to explode in mid-air shortly after liftoff, Yonhap news agency reported, citing unnamed sources. The U.S. military’s Indo-Pacific Command called it a “ballistic missile launch.” The command did not mention the reported failure, but condemned the launch and urged Pyongyang to refrain from further destabilising acts. A …