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The Pentagon announced that it will not support a proposal from NATO-ally Poland to send MiG-29 planes to the Ukrainian military, referring to the move as "high-risk" for escalating tensions with Russia.

Pentagon press secretary John Kirby told reporters that Defense Secretary Lloyd Austin spoke to Poland Defense Minister Mariusz Blaszczak on Wednesday about "Russia's unprovoked aggression in Ukraine," offering appreciation for the country's acceptance of Ukrainians who are fleeing turmoil.

Pentagon Press Secretary John Kirby holds a news briefing at the Pentagon on March 09, 2022 in Arlington, Virginia. Kirby spoke on various topics including Russia’s invasion to Ukraine.

Pentagon Press Secretary John Kirby holds a news briefing at the Pentagon on March 09, 2022 in Arlington, Virginia. Kirby spoke on various topics including Russia’s invasion to Ukraine. (Photo by Kevin Dietsch/Getty Images)

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During the conversation with his Polish counterpart, Austin, according to Kirby, made it clear that the U.S. does not support Poland’s proposal to deliver MiG-29s to the Ukrainian military.

"The secretary also had a chance to discuss with Minister Blaszczak the proposal to send MiG-29 fighter aircraft to Ukraine and, specifically, the notion of doing so by a way of transfer to U.S. custody," Kirby said. "Secretary Austin thanked the minister for Poland’s willingness to continue to look for ways to assist Ukraine, but he 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 either."

Kirby said the Pentagon believes the "best way to support Ukrainian defense is by providing them the weapons and the systems that they need most to defeat Russian aggression, in particular anti-armor and air defense."

FILE - Two Polish Air Force Russian made Mig 29's fly above and below two Polish Air Force U.S. made F-16's fighter jets during the Air Show in Radom, Poland, on Aug. 27, 2011. (AP Photo/Alik Keplicz, File)

Two Polish Air Force Russian made Mig 29's fly above and below two Polish Air Force U.S. made F-16's fighter jets during the Air Show in Radom, Poland, on Aug. 27, 2011. (AP Photo/Alik Keplicz, File)

Kirby said previous weapons sent to Ukraine from America and other nations are being "used with great effect."

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He said a defense assessment showed that "adding aircraft to the Ukrainian inventory is not likely to significantly change the effectiveness of the Ukrainian air force relative to Russian capabilities."

"Therefore, we believe that the gain from transferring those MiG 29s is low," Kirby said.

"The intelligence community has assessed that the transfer of MiG-29s may be mistaken as escalatory and could result in significant Russian reaction that might increase the prospects of a military escalation with NATO," Kirby said. "Therefore, we also assess the MiG-29s to Ukraine to be high-risk."

The Pentagon's response to the matter comes one day after Poland surprised the U.S. by announcing it would send all MiG-29 warplanes to the U.S. Ramstein Air Base in Germany. The Pentagon pushed back on the move and said it raised concerns regarding the deployment of warplanes from a U.S. base in a NATO-allied nation to combat Russian forces.

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Ukrainian President Volodymyr Zelenskyy on Wednesday pleaded with the U.S. and Poland to find a way to send his country MiG-29 warplanes as Russian forces continue to pummel Ukraine.

Volodymyr Zelenskyy

In this photo, Feb. 27, 2022, taken from video provided by the Ukrainian Presidential Press Office, Ukrainian President Volodymyr Zelenskyy speaks to the nation in Kyiv, Ukraine. (Ukrainian Presidential Press Office via AP, File)

"We are grateful to Poland … for its readiness to provide Ukraine with combat aircraft," Zelenskyy said in a virtual address. "The problem is in logistics. In fact, it is a technical issue. It must be solved — immediately."

Kirby warned during the briefing that America must "be careful about every decision we make" to ensure "that we aren’t making the potential for escalation worse."

Fox News' Jennifer Griffin and Caitlin McFall contributed to this article.