The United States on Tuesday rejected a surprise offer by Poland to send its MiG-29 fighter jets to Ukraine via a US air base in Germany.

Poland's foreign ministry announced on Tuesday that it was ready to deploy its MiG-29 jets to Ramstein Air Base in Germany and put them at the disposal of the United States. It urged other members of the alliance that had other such aircraft to do the same.

Under the proposed scheme, those jets could then be deployed to Ukraine, while the Polish air force would receive F-16 fighters as replacements.

Pentagon spokesman John Kirby said the prospect of the jets, placed at the disposal of the United States, flying from a US-NATO base "into airspace that is contested with Russia over Ukraine raises serious concerns for the entire NATO alliance."

"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," Kirby said in a statement.

"It is simply not clear to us that there is a substantive rationale for it," he added.

Kirby also emphasised that "the decision about whether to transfer Polish-owned planes to Ukraine is ultimately one for the Polish government to decide."

Ukraine's air force comprises aging Soviet-era MiG-29 and Sukhoi-27 jets, and heavier Sukhoi-25 jets which are the only planes Ukrainian pilots could fly immediately without additional training.

US Secretary of State Antony Blinken said during a visit to Moldova on Sunday that Washington was "actively" looking at a deal with Poland to provide Ukraine with the MiG-29s.

Ukrainian President Volodymyr Zelenskiy had made a plea for European countries to provide Russian-made planes, an issue he mentioned during a video call Saturday with U.S. lawmakers.

But after Poland announced its offer, US Undersecretary of State for Political Affairs Victoria Nuland told US lawmakers that Washington had been caught off guard.

Some U.S. lawmakers are eager to speed military aid to Ukraine and were reported to be pushing the Biden administration to facilitate the aircraft transfer.

Russia's defence ministry warned this week that countries offering air fields to Ukraine for attacks on Russia may be considered as having entered the conflict.

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