The United States has greenlit the transfer of F-16 fighter jets from Denmark and the Netherlands to Ukraine, a move that Russian officials had previously said would constitute “an unacceptable escalation.”

Secretary of State Antony Blinken notified the two NATO nations that the United States was signing off on the transfer in a letter, a copy of which was reviewed by Reuters.

“I am writing to express the United States’ full support for both the transfer of F-16 fighter aircraft to Ukraine and for the training of Ukrainian pilots by qualified F-16 instructors. … It remains critical that Ukraine is able to defend itself against ongoing Russian aggression and violation of its sovereignty,” Blinken wrote.

Although the transfer will not take place until the pilots have finished training, Blinken explained that pre-approval — before the planes and their crew are ready — will help avoid unnecessary delays so that Ukraine can take “full advantage of its new capabilities as soon as the first set of pilots complete [sic] their training, per Reuters.

Russian Minister of Foreign Affairs Sergey Lavrov said back in May, when the proposal of Western-supplied F-16s to Ukraine was first being contemplated, that Russia would consider such an act “an unacceptable escalation” and that the West was “playing with fire,” reported RT.

In June, Russian President Vladimir Putin warned that Russia would strike at bases outside Ukraine that hosted fighter jets engaged in the war.

President Joe Biden’s national security advisor, Jake Sullivan, told ABC News’ Good Morning America last month that the planes would “likely” come from “European countries that have excess F-16 supplies.”

On Wednesday, a Ukrainian air force spokesperson said that Ukraine does not foresee being able to deploy F-16s for combat at least until next year.

“It’s already obvious we won’t be able to defend Ukraine with F-16 fighter jets during this autumn and winter,” Yuriy Ihnat said on Ukrainian television, Reuters reported.

Western officials have reportedly maintained that even though they desire to see Ukraine achieve victory over Russia, they have no desire for a direct confrontation between NATO and Russia.