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.

With Russia seeking dominance in the skies over Ukraine with its jets that include Sukhoi Su-34 fighter-bombers (shown here), Ukraine was hoping the West would help boost its air forcen Defence Ministry on March 6, 2022 shows Russian Sukhoi Su-34 fighter
With Russia seeking dominance in the skies over Ukraine with its jets that include Sukhoi Su-34 fighter-bombers (shown here), Ukraine was hoping the West would help boost its air force n Defence Ministry on March 6, 2022 shows Russian Sukhoi Su-34 fighter-bomber takes off a missile strike amid Russia's ongoing military operation in Ukraine. Russian Defence Ministry via AFP

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.

Ukrainian President Volodymyr Zelensky, seen here in a handout video grab released by his office, has pleaded with the West for more air power in its fight against Russian forces, saying 'send us planes'
Ukrainian President Volodymyr Zelensky, seen here in a handout video grab released by his office, has pleaded with the West for more air power in its fight against Russian forces, saying 'send us planes' UKRAINE PRESIDENCY via AFP / Handout

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.

The Pentagon has rejected as too 'high risk' a plan that would have seen Poland transfer its fighter jets through a US air base in Germany to Ukraine to bolster that country's air power against Russia's invading forces
The Pentagon has rejected as too 'high risk' a plan that would have seen Poland transfer its fighter jets through a US air base in Germany to Ukraine to bolster that country's air power against Russia's invading forces GETTY IMAGES NORTH AMERICA via AFP / Kevin Dietsch

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

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."

US Vice President Kamala Harris disembarks from Air Force Two upon arrival in Warsaw, Poland  on March 9, 2022 to attend meetings about the conflict in Ukraine
US Vice President Kamala Harris disembarks from Air Force Two upon arrival in Warsaw, Poland on March 9, 2022 to attend meetings about the conflict in Ukraine POOL via AFP / SAUL LOEB

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.