The Sukhoi Su-27 incident in Ukraine involved a U.S. serviceman, the United States Air Forces in Europe - Air Forces Africa said in a statement.

"We have seen reports claiming a U.S. casualty and can confirm a U.S. service member was involved in this incident.

"It is currently under investigation and we will continue to provide more information as it becomes available."

It did not say how exactly the U.S. serviceman was involved in the incident. Nor did it clarify the fate of the U.S. pilot.

The Su-27UB fighter aircraft came down near the village of Ulaniv in Khmelnytsky district, Vinnytsia region, Ukraine, at around 5 p.m. The plane took off from an air base in Starokostiantyniv for a practice flight as part of the Clear Sky 2018 exercise.

Military prosecutors in Ukraine's Central region have launched an inquiry.

Both pilots are believed to be dead. One of them was a U.S. Air National Guard serviceman, the prosecutor's office said, before deleting this information.