Ukrainian President Volodymyr Zelensky fired Air Force Commander Mykola Oleschuk on Tuesday, a day after it was announced that one of the US F-16 fighter jets delivered to Kiev had crashed, killing its pilot.

“I decided to replace the commander of the Air Force of the Ukrainian Armed Forces (…) I will be eternally grateful to all our military pilots,” Zelensky himself said in a Telegram post after the presidential decree was made public.

The Ukrainian president did not specify the reasons for Oleschuk’s removal, but said that staff must be protected and that it is necessary to strengthen the administration.

Oleschuk’s duties will be temporarily taken over by Vice Admiral Anatoly Krivonozka.

The Ukrainian military has not specified the cause of the F-16 crash, saying only that it went down while approaching a Russian target. Oleschuk had said the US was helping to investigate the incident. A US official told Reuters the crash did not appear to be the result of Russian fire and was being investigated as pilot error or mechanical failure.