Ukraine’s biggest allies have so far said they won’t send fighter jets to Kyiv, denying a key request from President Volodymyr Zelenskyy ahead of an expected escalation in fighting.

President Joe Biden said Monday that the U.S. wouldn’t send F-16s to Ukraine, joining the U.K. and Germany, which have also ruled out sending their military fighters.

Kyiv’s allies have stepped up shipments of military hardware in recent weeks in an effort to bolster Ukraine’s forces ahead of a spring offensive that officials fear Russia may be planning. The U.S., Germany and the U.K. made U-turns earlier this month by promising to deliver heavy tanks to Ukraine.

“A victory of Ukraine without modern air force is scarcely imaginable,” Ukraine’s Deputy Foreign Affairs Minister Andrij Melnyk said on Twitter over the weekend. “That’s why we urgently need Western fighter jets like F-16 and F-35, as well as Eurofighter and Tornado. Let’s do it together, dear allies in DC and Berlin.”

Russia has failed to clinch air superiority over Ukraine, in part because it hasn’t invested in other lines of development like training, targeting systems, and command and control to give it a truly credible air force, Air Marshal Johnny Stringer, the deputy commander of NATO’s Allied Air Command, said in an interview late last year.

Despite Ukraine’s relative successes in fending off Russian air attacks, it could always benefit from more air defense systems to prevent further deadly barrages. The F-16s could potentially down cruise missiles like the ones Russia has launched at civilian infrastructure in recent months.

While there’s always a risk Russia could eventually gain control over Ukraine’s airspace, Stringer said the Ukrainian forces’ employment of their aircraft, their surface-to-air systems and the way they’ve managed to command them “gives me significant confidence that they’re going to be able to maintain this.”

The U.K. said it’s not practical to send fighter jets to Ukraine, even as British Prime Minister Rishi Sunak told his Cabinet Tuesday that Britain is accelerating its support to the war-torn nation.

“In terms of the U.K.’s fighter jets, these are sophisticated pieces of equipment so we do not think it’s practical,” Sunak’s spokesman, Max Blain, told reporters Tuesday, citing the length of time needed to learn to use “very complex pieces of equipment.”

Belgium has also said that it’s not planning to send any jet fighters to Ukraine.

“As far as the supply of F-16s is concerned, this question is not on the agenda,” Belgian Defense Minister Ludivine Dedonder said in emailed comments.

They are still in service with the Belgian Army and used for the protection of the Belgian, Dutch and Luxembourg airspace, she said.