Local residents reported seeing the jet lose altitude as it diverted from a civilian neighborhood, according to the Kiev-based Ukrainian Independent Information Agency of News, which quoted a Ukrainian air force statement.
The US Air Force identified the pilot who died during a routine training flight in Ukraine as Lt. Col. Seth "Jethro" Nehring. "The aircraft landed without incident back at Starokostiantyiv Air Base and is expected to resume participating in the exercise this week".
The Su-27UB fighter jet crashed near the western city of Khmelnytskyi, U.S. Air Forces in Europe said.
The exercises run until Friday, with United States fighter and transport planes training with about 30 Ukrainian aircraft.
The U.S. and Ukrainian governments are conducting an investigation to determine the cause of the accident.
"We are a close-knit family, and when a tragedy like this occurs, every member of the 144th Fighter Wing feels it", Col. Daniel Kelly, commander of the 144th, said in the release.
Earlier, the U.S. Air Force said it was aware of "reports claiming a U.S. casualty", but it did not confirm the death.
U.S. Air Force pilots are now in Ukraine participating in a large-scale multi-national aviation exercise known as "Clear Sky" involving close to 1,000 personnel from North Atlantic Treaty Organisation countries.
"We understand there was an American in the backseat of the aircraft", the statement said, adding that the Air Force is investigating the incident.
California units were heavily represented in Clear Sky with F-15C Eagles and C-130J Super Hercules. "It is now under investigation and we will continue to provide more information as it becomes available".