The Air Force said Nehring and Petrenko were on a routine familiarization training flight when their aircraft crashed in the Khmelnytskyi region of Ukraine, about 175 miles southwest of the capital city of Kyiv.
The General Staff of Ukraine's armed forces said the two-seat Su-27UB went down in the Kmelnitskyi region.
The nationalities of the victims were not immediately revealed.
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Officials said Wednesday they were waiting to release further details, such as the airman's job, until next of kin was notified.
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 circumstances of the crash are being investigated, the statement said. A Ukrainian pilot was also killed in the incident.
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They involve USA fighter and transport planes training with about 30 Ukrainian aircraft.
Clear Sky 2018 has otherwise been an immensely significant event for the pilots from the 144th Fighter Wing, who are among the approximately 950 personnel from nine countries, including the United States, who are taking part in the exercise. "It is now under investigation and we will continue to provide more information as it becomes available", it added.
U.S. Air Forces in Europe (USAFE) confirmed that an American military service member was aboard the Ukrainian Air Force SU-27 that crashed. The exercises, called Clear Sky-2018, include members of the California's National Guard's Fresno-based 144th Fighter Wing group, according to a Facebook post.
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She also called out actor and her former boyfriend Zain Durrani for physical abuse, and filmmaker Vikas Bahl for sexist remarks. She concluded saying that she does not endorse this behaviour and will stand in solidarity with any women who has been hurt.
California units were heavily represented in Clear Sky with F-15C Eagles and C-130J Super Hercules.