It was not immediately clear if the other pilot aboard the Su-27 was killed or injured.
During the combat training flight, the aircraft started to lose altitude and the pilots headed it away from residential districts to prevent civilian casualties, the General Staff said in a message published on Facebook on October 16, referring to locals who witnessed the crash.
"We have also seen the reports claiming a USA casualty and are now investigating and working to get more information".
According to the Ukrainian Military Portal, the accident took place during the multinational military exercise titled "Clear Sky-2018".More news: Kangaroos' new era marred by shock NZ loss
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"This is a sad day for the United States and Ukraine", Air Force Maj. "We will provide more information as soon as it becomes available".
The U.S. Air Forces in Europe and Africa has since confirmed that the U.S. service member, who has not been identified, was of the 144th Fighter Wing of California Air National Guard in Fresno.
Other U.S. aircraft participating in the exercise include California ANG C-130s sent to Vinnytsia Air Base; Illinois ANG KC-135s from Powidz Air Base, Poland, and active duty KC-135s from from RAF Mildenhall, England; as well as MQ-9 Reapers based at Miroslawiec Air Base, Poland. More details will be added as they emerge.
The Ukrainian General Staff had issued an earlier online statement indicating that both a Ukrainian pilot and an American pilot were on board and killed in the crash.