The crash marks a major upset for Kyiv, which has been looking to secure advanced F-16 jets to strengthen its Soviet-era air force and push ahead with its counteroffensive.

"We express our condolences to the families of the victims. This is a painful and irreparable loss for all of us," Ukraine's air force said, calling the crash "terrible".

The crash took place in Ukraine's northern Zhytomyr region and involved two L-39 combat training aircraft, it said, adding an investigation was underway.

Air force spokesperson Yuriy Ignat said one of the pilots killed -- Andrii Pilshchykov -- who went by the call-sign "Juice", was a young officer with "mega knowledge and mega talent".

"Major Andrii Pilshchykov devoted his short but very bright life to combat aviation, he dreamed of F-16s in the Ukrainian sky," he said in a tribute shared on social media.

"You were more than a friend. Rest in peace, you have done so much for us," he added.