A Russian fighter jet flew dangerously close to a US spy plane over the Black Sea on Wednesday, coming within just 10 feet of the American aircraft, a US defense official said.

"During the intercept, which lasted approximately 19 minutes, the SU-27 (Flanker jet) initially maintained a 30-foot separation distance, then closed to within 10 feet (3 meters) of the P-8A (Poseidon), which is considered unsafe and unprofessional," the official said, speaking on condition of anonymity ahead of a Pentagon announcement.