WARSAW, Poland — In a private video call with American lawmakers over the weekend, Ukrainian President Volodymyr Zelenskyy made a “desperate” plea to the United States to help Kyiv get more warplanes to fight Russia’s invasion and retain control of its airspace.

U.S. Secretary of State Antony Blinken says Washington has given a “green light” to the idea and is currently “very, very actively” looking at a proposal under which Ukraine’s neighbor Poland would supply Kyiv with Soviet-era fighters and in turn receive American F-16s to make up for their loss.

However, the proposition is fraught with uncertainty and Poland has been less than enthusiastic about it in public, largely because Russia has warned that supporting Ukraine’s air force would be seen in Moscow as participating in the conflict and open up suppliers to possible retaliation. Official comment from NATO and European Union member Poland has been only to confirm continuing talks on the subject.

WHY DOES UKRAINE NEED WARPLANES?

Ukraine’s air force uses Soviet-made Mig-29 and Su jet fighters to defend its skies and territory from Russia’s military invasion that began Feb. 24 and has appealed for more warplanes to be able to continue the mission in the long run.

The air force has been far outnumbered by the much more powerful Russian air force, but Ukrainian pilots have continued to fly combat sorties and claim kills in combat despite repeated assertions by the Russian military that it has suppressed Ukraine’s air power and air defense assets.

WHY NOT US WARPLANES?

Ukraine’s military pilots aren’t trained to fly U.S. jet fighters and would be far more equipped to handle MiG-29 or Su planes that are currently used by former Soviet-bloc NATO members Poland, Bulgaria and Slovakia.

Ukrainian pilots would be able to fly MiGs right away, but Poland isn’t eager to lose significant amounts of its air force without replacements. U.S.-made F-16s are becoming the mainstay of Poland’s air force as it modernizes its military.

WHAT IS POLAND’S RESPONSE?

Blinken has said there is a “green light” for Poland to send planes to Ukraine.

“We are looking actively now at the question of airplanes that Poland can provide to Ukraine and looking at how we might be able to backfill should Poland decide to supply those planes. I can’t speak to a timeline, but I can just tell you we’re looking at it very, very actively,” Blinken said on Sunday in Moldova.

The response from Poland was restrained, though.

“As far as sending planes, I can only repeat that no decisions have been taken on the subject,” government spokesman Piotr Mueller said.

Mueller denied allegations that Poland could be making its airfields available to Ukrainian warplanes. Russia alleges that Romania and some other countries it didn’t name are hosting Ukraine’s warplanes.

Meanwhile, Poland has been supportive of Ukraine both politically, supporting it’s territorial integrity and sovereignty, and on a humanitarian level, opening its border to refugees from the non-EU country.

WHY IS POLAND NON-COMMITAL?

Despite it’s supportive stance toward Ukraine in its struggle, Warsaw is facing a crucial and challenging decision regarding making its planes available to Ukraine.

Russia has warned Ukraine’s neighbors against hosting its warplanes on their territory saying Moscow could consider that as their “engagement in the military conflict.” That could mean an opening of hostilities.

Russia’s words could be taken as a wider warning against aiding Ukraine’s air forces.

Poland also borders Russia, through the Kaliningrad exclave, and has a long border with Russia’s close ally Belarus. Relations between Warsaw and Moscow have been at a low point since a right-wing government took office in Poland in 2015.

OTHER CONSIDERATIONS

One of the main issues is where these MiGs, if made available, would be based as they couldn’t be on NATO soil. It isn’t clear if Ukraine would be able to safely house and service them in the long run, given the warfare on its territory.

Another question to resolve would be how to deliver the planes to Ukraine. Polish pilots, who are also NATO pilots, couldn’t fly them to Ukraine without risking NATO involvement in the conflict, and sending Ukrainian pilots to Poland to fly them back could present similar issues.

There is also an F-16 production backlog, which means the countries that potentially give away their MiGs and Su fighters to Ukraine would need to wait for the backfill for some time.

U.S. Senator Marco Rubio summed it up by saying: “There’s complications that come with. It’s not just as easy as handing it over. You got to fly those in. You got to station them somewhere on the ground.

“And … the Russians have launched a pretty — anywhere in between eight to 12 rockets at an airport in the sort of west of Ukraine. And it’s just a part of a strategy to deny them places to move that airframe,” said Rubio, a Republican from Florida.