Warsaw caught US officials off guard with the offer to deliver the Soviet-era planes to the US base in Ramstein, Germany.

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 stressed that Washington's stance was that "the decision about whether to transfer Polish-owned planes to Ukraine is ultimately one for the Polish government."

Ukraine has stepped up calls for Western allies to supply it with military jets in the face of the Russian invasion, but providing Kyiv with war planes poses serious risks -- with NATO members unwilling to be deemed co-combatants by Moscow.

Ukraine's air force fleet consists of aging Soviet-era MiG-29 and Sukhoi-27 jets, and heavier Sukhoi-25 jets -- and these 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.

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.

Asked by a senator whether US officials coordinated ahead of time with Warsaw, Nuland said: "Not to my knowledge."

While several lawmakers pressed for the jets to be rushed to Ukraine, Nuland refused to commit Washington to supporting or facilitating the exchange.

"I will continue to convey the very strong bipartisan view of this committee that these planes need to get to Ukraine," she told the panel.

"There are a number of factors to consider here and there are some mixed views among allies and even within the administration."

While a significant part of Ukraine's air force remains intact since the war began on February 24, both Ukraine and Russia have sustained significant losses and neither controls the airspace over the country.