The Pentagon on Wednesday conclusively rejected as too "high risk" a plan to transfer fighter jets from Poland to Kyiv to battle Russian forces, pouring cold water for now on Ukraine's bid for more firepower in the skies.

The scheme -- which the United States said it was actively studying before nixing it over concerns the Kremlin could interpret it as a military escalation -- has proven to be a notable hiccup in the otherwise solid alliance between Washington and European allies over Moscow's invasion.

US Defense Secretary Lloyd Austin spoke with Poland's Defense Minister Mariusz Blaszczak earlier Wednesday and thanked him for Warsaw's willingness to cooperate in the effort to assist Ukraine, Pentagon spokesman John Kirby told reporters.

But during the call, Austin "stressed that we do not support the transfer of additional fighter aircraft to the Ukrainian Air Force at this time, and therefore have no desire to see them in our custody," Kirby said.

Warsaw had expressed support for a plan in which Poland would have sent its Soviet-era MiG-29s to Kyiv via a US air base in Ramstein, Germany, with the suggested follow-on step that the United States would backfill Poland's fleet with American F-16 fighter jets.

"That is something that we are not going to explore right now," Kirby said, adding that adding aircraft to Ukraine's fighter fleet was "not likely to significantly change the effectiveness" of the Ukrainian Air Force against Russian capabilities.

Kirby said intelligence agencies had warned that the MiG transfer to Ukraine "may be mistaken as escalatory and could result in significant Russian reaction that might increase the prospects of a military escalation" with the NATO alliance.

"Therefore, we also assess the transfer of the MiG-29s to Ukraine to be high risk."

Indeed Moscow warned against the scheme.

"This is a highly undesirable and a potentially dangerous scenario," Kremlin spokesman Dmitry Peskov said Wednesday.

 'Send us planes'

The Pentagon spokesman insisted the best way to support Ukrainian defense is by providing them with weapons systems they need the most, particularly for air defense, that the United States and other nations continue to send Kyiv, and which are "being used with great effect."

That's cold comfort for a Ukraine defending itself against one of the world's premier military powers, as President Volodymyr Zelensky called in vain Wednesday for the jets debate to be favorably resolved.

"When will there be a decision? Look, we're at war!" he said in a video on his Telegram channel. "Send us planes."

Zelensky made the remarks with media reports swirling, after the Pentagon's preliminary rejection Tuesday of the transfer.

"We don't have time for the media, for all these signals," he added. "This isn't ping-pong. It's human lives."

The White House sought to paper over the dispute, with press secretary Jen Psaki describing it as "a temporary breakdown in communications" with the Poles.

Other Western powers also had expressed serious reservations.

German Chancellor Olaf Scholz, speaking in Berlin alongside Canadian Prime Minister Justin Trudeau, signalled his opposition to the jet offer.

Germany has given "significant" financial support to Ukraine, as well as offering humanitarian aid and some weapons, he said.

"Apart from that, we have to think very carefully about what we are doing, and this certainly does not include fighter jets," Scholz added.

Trudeau warned of the risk of "expanding or escalating" the war, saying that while Canada is eager to support Ukrainians, "we need to be mindful of the best way to support them."

Washington meanwhile announced it sent two Patriot surface-to-air missile batteries to Poland to defend against any "potential threat" to US or NATO forces in the alliance's territory.

And Vice President Kamala Harris arrived in the country late Wednesday to attend meetings about the war in Ukraine.