A top Ukrainian pilot was killed when a US-made F-16 fighter jet crashed on Monday, a source in the Ukrainian defense forces told CNN.

The Ukrainian defense forces do not believe that behind the incident there is pilot errorthe source clarified.

The pilot, named Oleksii Mes, known as “Moonfish”, was killed in the crash while “repelling the largest ever air attack” by Russia against Ukraine, the Ukrainian source said, adding that the pilot was buried on Thursday.

The crash is being investigated and international experts will be invited to join the investigation, the source added.

The death of the pilot it’s a big blow for Ukraine. The first F-16s arrived in the country earlier this month, and “Moonfish” was one of the few pilots trained to fly them.

Kiev has waited a long time to acquire the F-16s, with Ukrainian President Volodymyr Zelensky asking his Western allies for the fighter jets since the start of the full-scale Russian invasion of his country.

But as with other equipment, Western countries hesitated before finally agreeing to provide F-16s. The Netherlands and Denmark pledged to provide them with the fighters in the early summer of 2023, but it took a few more weeks before the US gave the go-ahead for their transfer.

When they arrived earlier this summer, Zelensky said he and his government held “hundreds of meetings and negotiations” to secure the planes.

A group of Ukrainian pilots began training in the US in the fall. While usually needed one year to fully train a pilot to fly the planes, Moonfish and the other operators had to do so in just six months.