Pentagon spokesman John Kirby said Poland's declaration that it intended to deliver the 28 jets to the US Ramstein Air Base in Germany raised the concerning prospect of warplanes departing from a US and NATO base to fly into airspace contested with Russia in the Ukraine conflict.

"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.

The proposed gift of more warplanes would be a morale booster for Ukrainians now under Russian assault for nearly two weeks. But it also raises the risk of the war expanding beyond Ukraine.

Russia has declared that supporting Ukraine's air force in this way would be tantamount to participating in the war, and would possibly spur retaliation.

White House officials were blindsided by the Polish government announcement on the MiGs. The proposal did not come up during talks with Secretary of State Antony Blinken when he was recently in Poland, according to a US official familiar with the talks.

The US official, who spoke on the condition of anonymity, said White House officials did not think the proposal would easily solve the logistical challenges of providing aircraft to Ukraine.

US Undersecretary of State Victoria Nuland told politicians at a Senate Foreign Relations Committee hearing on the Ukraine crisis on Tuesday she learned of Poland's plans only while driving to the hearing.

"To my knowledge, it wasn't pre-consulted with us," Nuland told senators.

Ukraine has been pleading for more warplanes as it puts up a tough resistance against stronger Russian forces. Washington has been looking at a proposal under which Poland would supply Ukraine with the Mig29s and in turn receive American F-16s to make up for their loss. Ukrainian pilots are trained to fly the Soviet-era fighter jets.

The Polish Foreign Ministry announced the plan in a statement, which said the jets would be delivered to Ramstein free of charge.

Poland's decision to publicly float its plan came the day before US Vice President Kamala Harris was scheduled to depart for Warsaw for talks with Polish officials. The disconnect is likely to add an awkward layer to the talks, which were expected to focus largely on US efforts to help Poland and other eastern European nations that have taken in some 2 million refugees since the war started.