The Russian invasion of Ukraine has prompted Western nations to supply an ever-growing list of weaponry to Kyiv as it seeks to defend itself: small arms to anti-tank weapons to artillery to missiles to tanks.
Such expansions — particularly the deal this month to begin supplying Ukraine with German- and U.S.-made tanks — has promised equipment that previously seemed off limits.
So what about Ukrainian officials’ calls for some of its allies’ most potent weapons: military jets?
A top adviser to Mr. Zelensky, Andriy Yermak, suggested on Monday that Ukraine had begun pressing NATO countries on the question of warplanes, saying on Telegram that Kyiv had received “positive signals” from Poland about F-16 fighter jets. Poland, an early advocate of sending German-made tanks to Ukraine, has stressed that it coordinates weapons decisions with other NATO members.
And Wopke Hoekstra, the foreign minister of another NATO member, the Netherlands, recently told Dutch lawmakers that the government would be willing to send American-made F-16 jets if the United States authorized the transfer.
However, on Monday, President Biden, asked by a reporter whether the United States would provide F-16 fighter jets, said it would not. The White House declined to comment on a question about whether Mr. Biden was ruling out the use of the jets entirely or just an immediate transfer of them.
Other leaders have been more direct. Chancellor Olaf Scholz of Germany reiterated recently that Berlin would not send fighter jets to Ukraine. “That we are not talking about fighter aircraft is something I made clear very early on, and I’m making that clear here as well,” he said in an announcement that Germany would send Ukraine tanks.
On Monday, Britain’s defense minister, Ben Wallace, acknowledged the questions about aircraft in remarks to members of Parliament.
“Since we took on the battle over getting tanks to Ukraine, people are understandably asking what will be the next capability,” he said. “What we know about all these demands is that the initial response is no, but the eventual response is yes.”
Britain, Mr. Wallace said, would track the progress of discussions among Western allies, but noted that decisions about military aid are not “an ad hoc thing.”
Last week, the U.S. stance seemed flexible. A Pentagon spokeswoman, Sabrina Singh, said then that she didn’t believe the United States had ever “drawn a line” over the arms it was willing to supply, and emphasized that the U.S. was providing Ukraine with significant air-defense capabilities.
But should Western nations provide advanced aircraft, training for Ukrainian pilots would be a complicating factor, she said, requiring “more people to come off the battlefield to learn an entirely new system.”
Were fighter jets to be sent, Ukrainian pilots would not be the only ones needing training. The logistics needed to support a tranche of aircraft unfamiliar to Ukrainian mechanics, who are trained on Soviet-era equipment, would be extensive and time consuming.
And just how such aircraft would be utilized remains an open question. The proliferation of surface-to-air missiles on both sides has ensured that air combat and bombing runs are rare compared to the grinding artillery battles that have come to define the war.
The United States’ supply of AGM-88 HARM anti-radar missiles that began arriving over the summer has allowed Ukraine’s Air Force — primarily composed of Soviet-era jets and helicopters — to fire their ordnance far enough away from the front lines as to not be exposed to Russian air defenses.
A supply of new jets “would reduce Ukraine’s disadvantage versus the Russian Air Force, and simplify the use of Western air-launched munitions, but this is a lower priority issue, all things considered,” said Michael Kofman, the director of Russian studies at C.N.A., a research institute in Arlington, Va.