A Ukrainian military jet crashed Tuesday during a North Atlantic Treaty Organization training exercise, killing two people, military officials said.
The identity of the American was being withheld for 24 hours to allow for the notification of family.
Officials said Wednesday they were waiting to release further details, such as the airman's job, until next of kin was notified.
About 950 service members from nine militaries are taking part in the exercise, including the United States, United Kingdom, Ukraine, Belgium, Denmark, Estonia, the Netherlands, Poland and Romania. Gen. Clay Garrison, California ANG commander and Clear Sky exercise director, said in a statement.
"We share in the sorrow felt by Jethro's loved ones and our thoughts and prayers go out to his family and friends as well as those of the Ukrainian aviator".
The pilot was assigned to the 144th Fighter Wing based at Fresno, California, said Capt. Christopher Bowyer-Meeder, a spokesman for U.S. Air Forces in Europe. The U.S. and Ukrainian governments are continuing to investigate the incident.
Clear Sky began October 8 and is scheduled to end this week. The goals of the exercise were to enhance regional security and cooperation between Ukraine and North Atlantic Treaty Organisation allies, and other partners in the region.
Kiev, which is not a member but wants to join North Atlantic Treaty Organisation, and the West have accused Russian Federation of backing rebels in Ukraine's war-torn east and sending troops across the border, claims Moscow has repeatedly denied.
In recent years, concerns have risen about Russia's intentions in Ukraine, particularly after Moscow's annexation of the Crimean peninsula in 2014. It is the first time ever that the unit has brought its F-15C Eagles to Ukraine and is the first time the Air Force has deployed this kind of aircraft to the country since 1998, according to Military.com.