The United States has rejected a Polish offer to send MiG-29 fighter jets to Ukraine via a US air base, saying the proposal raised "serious concerns" for the entire NATO alliance.
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," Mr Kirby said in a statement.
"It is simply not clear to us that there is a substantive rationale for it," he added.
Mr 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.
Surface to air missile batteries
The United States has sent two manned Patriot surface to air missile batteries to an unspecified location in Poland.
A senior Pentagon official said that the batteries were to defend against any "potential threat" to United States or NATO forces in the alliance's territory.
Normally stationed in Germany, the Patriot surface to air missile defense system is capable of countering and destroying ballistic missiles, cruise missiles or aircraft.
The missile batteries were repositioned at Poland's invitation and the move is seen as reflecting growing fear that a Russian missile could cross the border from neighboring Ukraine into Poland.
The Pentagon official said the missile batteries were sent to Poland as a "purely defensive deployment" and that this was "100 percent in keeping with the seriousness with which the United States take our Article 5 commitments" to defend fellow members of the transatlantic alliance.
Vice President of the United States Kamala Harris is travelling to Poland to continue talks on how to best provide military assistance for Ukraine.
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 politicians that Washington had been caught off-guard.
Asked by a senator whether US officials co-ordinated ahead of time with Warsaw, Ms Nuland said: "Not to my knowledge."
While several politicians pressed for the jets to be rushed to Ukraine, Ms 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 24 February, both Ukraine and Russia have sustained significant losses and neither controls the airspace over the country.
