Ukraine has sought U.S.-made F-16 fighter jets to help it counter Russia's air superiority for more than two years of war. The source did not say which country would supply the jets.

So far, Denmark, the Netherlands, Norway and Belgium have committed to sending F-16s to Ukraine.

Illya Yevlash, spokesperson for the air force, said this week that some Ukrainian pilots were completing their training to fly the warplanes.

The pilots and ground staff have been trained by Ukraine's Western partners for months.

The Ukrainian military has had to rely on a relatively small fleet of Soviet-era jets as it has fought to hold back Russia's full-scale February 2022 invasion.

With Russian forces slowly advancing in the eastern Donbas region and mounting a fresh assault in the northeast near the city of Kharkiv, Ukrainian officials see the addition of the F-16 as a vital upgrade for its Air Force.