LONDON — A Russian fighter jet fired a missile at a manned British surveillance aircraft flying over the Black Sea in September but the munition malfunctioned, according to U.S. defense officials and a recently leaked classified U.S. intelligence report. The incident was far more serious than originally portrayed and could have amounted to an act of war.

According to two U.S. defense officials, the Russian pilot had misinterpreted what a radar operator on the ground was saying to him and thought he had permission to fire. The pilot, who had locked on the British aircraft, fired, but the missile did not launch properly.