WASHINGTON (Reuters) — U.S. lawmakers pushed President Joe Biden’s administration on Monday to facilitate the transfer of fighter aircraft to Ukraine from Poland and other NATO and Eastern European countries, after a plea on Saturday from Ukrainian President Volodymyr Zelenskiy.

The White House said it did not oppose planes being sent to Ukraine but saw logistical challenges to it.

Senate Foreign Relations Committee Chairman Bob Menendez wrote to Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin calling for Washington to commit to replace any donated jets with upgraded Western aircraft, including through concessionary financing and loans as well as subsidized pricing.

Zelenskiy made a “desperate plea for European countries to provide Russian-made planes” for Ukraine’s fight against Russian invaders during a video call Saturday with U.S. lawmakers, participants in the call said.

Many air forces in Eastern Europe fly Russian-made warplanes, and transferring such aircraft to Ukraine would mean Ukrainians could pilot the planes without additional training.

“I will support efforts in the Senate to implement measures to compensate our allies that provide their aircraft for Ukraine’s defense,” Menendez wrote.

A State Department spokesperson said the department was working with Poland and consulting with other NATO allies.

“This is Poland’s sovereign decision to make. We have in no way opposed Poland transferring planes to Ukraine,” the spokesperson said in an email.

Administration officials said there were logistical challenges both in moving aircraft into Ukraine, and providing replacement U.S. planes.

“It is not as easy as just moving planes around,” White House press secretary Jen Psaki said.

Many lawmakers – both Biden’s fellow Democrats and Republicans – have appealed for Washington to do all it can to get aircraft to Ukraine, including by letting countries that send Russian-made MiGs “jump the line” to quickly obtain U.S.-made F-16s – made by Lockheed Martin LMT.N – to replace them.

Russia describes its actions as “a special military operation” whose aim is to disarm Ukraine, counter what it views as NATO aggression and capture Ukrainian leaders it calls neo-Nazis.