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Vice President Kamala Harris and Polish President Andrzej Duda insisted Thursday that the United States and Poland are "unified" after the U.S. rejected the country’s proposal to send MiG-29 planes to the Ukrainian military.

During a joint press conference in Warsaw, Poland, just hours after the Pentagon made clear that the U.S. does not support Poland’s proposal for the U.S. and NATO to deliver MiG-29s to the Ukrainian military, Harris and Duda maintained that the relationship between the United States and Poland has "become even stronger" amid Russia's war against Ukraine

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Harris said her visit to Poland is "an expression of the enduring and important relationship between the United States and Poland that again has been long-standing, but in particular on the issue of Ukraine is unified and is clear." 

Vice President Kamala Harris speaks during a joint press conference with Poland's President Andrzej Duda on the occasion of their meeting at Belwelder Palace, in Warsaw, Poland, Thursday, March 10, 2022. (Saul Loeb/Pool Photo via AP)

Vice President Kamala Harris speaks during a joint press conference with Poland's President Andrzej Duda on the occasion of their meeting at Belwelder Palace, in Warsaw, Poland, Thursday, March 10, 2022.  (Saul Loeb/Pool Photo via AP)

"We will do everything together in partnership, in solidarity, to support what is necessary at this very moment in terms of the humanity and security needs of Ukraine and the Ukrainian people," Harris said. 

"I want to be very clear: The United States and Poland are united in what we have done and are prepared to do to help Ukraine and the people of Ukraine — full stop," Harris said.

Harris highlighted the U.S. House of Representatives' vote to send $13 billion in aid to Ukraine and European allies. And she announced the U.S. would provide $53 million more to the UN World Food Program.

Polish President Andrzej Duda speaks during a joint press conference with Vice President Kamala Harris on the occasion of their meeting at Belwelder Palace, in Warsaw, Poland, Thursday, March 10, 2022. (Saul Loeb/Pool Photo via AP)

Polish President Andrzej Duda speaks during a joint press conference with Vice President Kamala Harris, in Warsaw, Poland, Thursday, March 10, 2022.  (Saul Loeb/Pool Photo via AP)

Poland surprised the U.S. on Tuesday by offering to give its entire fleet of MiG-29 fighter jets to the U.S. in exchange for a chance to buy American F-16s as part of a deal to bolster the Ukrainian air force while upgrading the Poles’ with NATO aircraft.

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"The authorities of the Republic of Poland, after consultations between the President and the Government, are ready to deploy – immediately and free of charge – all their MIG-29 jets to the Ramstein Air Base and place them at the disposal of the Government of the United States of America," the Polish Foreign Ministry said in a statement Tuesday. "At the same time, Poland requests the United States to provide us with used aircraft with corresponding operational capabilities. Poland is ready to immediately establish the conditions of purchase of the planes."

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The Pentagon, the following day, 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.

U.S. Vice President Kamala Harris is greeted by Polish President Andrzej Duda during their meeting in Warsaw, Thursday, March 10, 2022. (AP Photo/Czarek Sokolowski)

Harris is greeted by Polish President Andrzej Duda during their meeting in Warsaw, Thursday, March 10, 2022.  (AP Photo/Czarek Sokolowski)

Duda, responding to questions on whether his government "unilaterally" made the offer to Ukraine, explained Poland was acting in the best interest of NATO. 

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"There is a fight going on for independence for sovereignty, for freedom and for survival and support with all the strength we have for the people of Ukraine," Duda said. "It is not only in the military, also the people of Ukraine have taken the arms to defend their homeland. It's not only men, it's also women fighting on the front line with men in order to defend their country." 

Duda said that Poland and its allies "accept" that Ukraine has "the right to demand this assistance from the international community." 

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.

With regard to those demands, Duda said he is making decisions "not only through our own lens of Poland," but also is looking to "adopt in the perspective of the security of NATO as a whole." 

"In a nutshell, we have to be a member of the alliance," he said, adding that in receiving requests from Ukraine, and "the media," Poland "behaved in such a way as a reliable member of NATO should serve NATO, who does not want to expose NATO to any difficult situation." 

Duda stressed that as a member of NATO, Poland "wanted to look for ways of support for the fighting of its allies." 

"And that is why we formulated that statement together with the Polish government and myself as president, together, with the Polish government," he said, adding that the statement discussed "the Polish civilities military support of assisting Ukraine." 

"Simply, we decided to put those jets at the disposal of NATO's not expecting anything in return because we stressed very clearly that as a gap-filler for the donated equipment we are able to buy what we would need as a replacement," Duda said. "And we, ourselves, were ready to do that and provide our equipment free of charge and that we wanted NATO, as a whole, to make a common decision so that Poland remains a member of NATO, not a country who decides on its own on important issues, which impact the security of NATO's help, which would impact this initiative of all members of NATO. Also, our neighbors who are part of NATO, members of NATO." 

Pentagon spokesman John Kirby said the Department of Defense 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."

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

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."

Ukraine has been pleading with Western allies to send military aid and implement a no-fly zone over the country. The U.S. and its NATO allies have repeatedly said a no-fly zone over Ukraine could bring about a direct conflict with Russia.

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Meanwhile, during her time in Poland, officials said the vice president will engage with Ukrainian refugees. More than 1 million refugees have fled from Ukraine to Poland since Russia invaded on Feb. 24. She is also expected to meet with Embassy Warsaw staff, as well as Embassy Kyiv staff who have relocated to Poland amid Russia’s invasion of Ukraine.

Harris, on Friday, is set to meet with U.S. and Polish service members in Warsaw to thank them for their service, before traveling to Bucharest, Romania, for a bilateral meeting with Romanian President Klaus Iohannis and members of Embassy Bucharest staff.

Fox News' Kyle Morris, Jennifer Griffin, Caitlin McFall and The Associated Press contributed to this report.