WASHINGTON — A member of the California Air National Guard’s 144th Fighter Wing and a Ukrainian service member died in a Oct. 16 crash of an Ukrainian Su-27 aircraft flying in Ukraine’s Clear Sky exercise, the U.S. Air Force said Wednesday.

The U.S. airman was taking part in a single-aircraft familiarization flight with a Ukrainian pilot, and no other aircraft were involved in the incident, according to a news release put out by U.S. Air Forces Europe. The Air Force has not released the identity of the guardsman yet, but intends to release it in 24 hours after kin are notified.

“This is a sad day for the United States and Ukraine,” said Maj. Gen. Clay Garrison, California ANG commander and Clear Sky exercise director. “Our deepest condolences go out to the family, friends and fellow airmen of both the U.S. airman and Ukrainian aviator who were killed in the incident.”

Shortly after the crash occurred approximately 5:00 p.m. local time in the Khmelnytskyi region of western Ukraine, news media released reports, citing the Ukrainian general staff, that claimed two pilots — one Ukrainian and one U.S. Air National Guardsman — had died during the accident. However, the post had since been removed, raising speculation that its initial statement had been incorrect.

In a statement made Tuesday, the U.S. Air Force was not immediately able to confirm any casualties.

“We have seen reports claiming a U.S. casualty and can confirm a U.S. service member was involved in this incident,” U.S. Air Forces Europe said in a Tuesday statement. “It is currently under investigation and we will continue to provide more information as it becomes available.”

The Su-27 crash casts a pall on the first-ever Clear Sky, which kicked off this month and aims to increase the interoperability of NATO countries and partner nations’ air forces through training that includes air sovereignty, cyber defense, air-to-ground integration and aeromedical evacuation, among other missions.

About 950 servicemembers from nine militaries are taking part in the exercise, including the United States, United Kingdom, Ukraine, Belgium, Denmark, Estonia, the Netherlands, Poland and Romania.

California Air National Guard F-15s were deployed to Starokostiantyniv Air Base in Ukraine to train alongside Ukrainian fighters, reported Air Force Times earlier this month.

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.

Updated 10/17/18 at 9:27 a.m. with additional information about the pilots' condition.