Poland announced Tuesday that it would hand over all of its MiG-29 fighter aeroplanes to the United States, potentially furthering an agreement that would allow the fighters to be passed on to Ukraine’s military as it battles invading Russian forces.

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The Pentagon rejected Poland’s proposal to transfer MiG-29 fighter jets to the US for delivery to Ukraine, calling it not “tenable,” according to Pentagon press secretary John Kirby on Tuesday.

The Pentagon is in contact with the Polish government about the problem, but Poland’s proposal demonstrates the “complexities” of deploying the fighters to Ukraine, according to Kirby’s statement.

“It is simply not clear to us that there is a substantive rationale for it,” Kirby said. “We will continue to consult with Poland and our other NATO allies about this issue and the difficult logistical challenges it presents, but we do not believe Poland’s proposal is a tenable one.”

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As per the foreign ministry statement, the Polish government proposed relocating all of their MiG-29s to the US Air Force’s Ramstein Air Base in Germany. According to the ministry, the US government would then provide them to Ukraine. Poland, in exchange, demanded secondhand fighter jets to replace the MiG-29s.

The unexpected statement by Poland complicates what was already a high-stakes visit by US Vice President Kamala Harris.

According to officials, Harris was scheduled to discuss the fighter jet issue while in Poland. The White House earlier stated that it was in talks with the Polish government on a proposal in which Poland would supply Ukraine with Soviet-era fighter aeroplanes and the US would backfill the planes with F-16s.

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Harris left for Poland on Wednesday morning, but there are now intense discussions within the administration about how to deal with Poland to establish an arrangement that allows the jets to reach Ukraine.

Ukraine has been clamouring for more jets, and Washington is considering a proposal in which Poland would supply Ukraine with Soviet-era fighters in exchange for American F-16s to compensate for the loss. Ukrainian pilots are trained to operate fighter jets from the Soviet era.

In a statement, the Polish Foreign Ministry stated that Poland was ready to give the jets to the US Ramstein Air Base in Germany promptly and free of charge.