UNITED NATIONS —President Donald Trump offered an optimistic assessment of Ukraine's prospects after meeting with Volodymyr Zelenskyy at the United Nations, saying he believes the country can retake all its territory from Russia after previously asserting that territorial concessions would be a necessity.

Trump claimed Ukraine "is in a position to fight and WIN all of Ukraine back in its original form."

"With time, patience and the financial support of Europe and, in particular, NATO, the original Borders from where this War started, is very much an option. Why not?" Trump said on social media, adding that Russia looked weak after "fighting aimlessly for three and half years."

Trump said earlier that NATO countries should shoot down Russian aircraft that violate their air space but said U.S. involvement in a potential conflict would depend on the circumstances.

The president's about-face on Ukraine's ability to win the war and support for NATO nations such as Poland that have had their airspace breached came after a day of meetings with U.S. allies during the United Nations General Assembly gathering Sept. 23.

Zelenskyy told Trump there was "good news" on the battlefield and thanked him for selling NATO weapons that are being transferred to Ukraine.

On social media, Trump — who once berated Zelenskyy in the Oval Office — praised him as "a great man" who is "putting up one hell of a fight." After the meeting, Trump jabbed at Russia as a "paper tiger" and said on social media that the United States would continue to funnel weapons to Ukraine through NATO.

Trump's comments about Ukraine regaining its territory are a shift from what he said in the run-up to a meeting in Alaska last month with Russian President Vladimir Putin. Trump rolled out the red carpet for the Russian leader in the hopes that a face-to-face meeting could generate a breakthrough in peace talks.

Putin is seeking full authority over four Ukrainian regions – Luhansk, Donetsk, Zaporizhzhia and Kherson – and the Black Sea peninsula of Crimea, which he annexed in 2014. His forces do not currently control all the territory in the regions.

Last month, after he met with Putin, Trump said Ukraine would have to engage in some "swapping of territories." Zelenskyy has rejected any territorial concessions.

The Alaska summit ended without a deal. And since then, Putin has continued to pound Ukraine, to Trump's frustration.

Before meeting with Zelenskyy, Trump delivered a speech to the General Assembly in which he threatened Russia with tariffs and urged European nations to adopt stiff economic measures to force an end to the war.

"In the event that Russia is not ready to make a deal to end the war, then the United States is fully prepared to impose a very strong round of powerful tariffs, which would stop the bloodshed, I believe, very quickly," Trump said. "But for those tariffs to be effective, European nations... would have to join us in adopting the exact same measures."

Trump has said he’ll put tariffs on nations that are purchasing Russian oil and gas as soon as the European Union and NATO allies fully cut Moscow off. Those efforts have been hampered by Slovakia, Hungary, and Turkey, which are still heavily reliant on Russian oil.

European Commission President Ursula von der Leyen told reporters later, during a meeting with Trump, that the bloc would put sanctions on ports where fossil fuels are coming from Russia. “We want to put tariffs on oil supplies that are still coming to the European Union," she said. "We want to get rid of it."

She said the EU has "to be faster" than a deadline it set for 2027.

Trump endorses NATO countries shooting down Russian aircraft

Trump also said during his meeting with Zelenskyy that NATO countries should shoot down Russian aircraft that violate their air space.

Responding to a question, he said, "Yes I do." But asked if the United States would help the countries if they engage Russian aircraft, Trump said it "depends on the circumstance," without elaborating.

His comments came after Poland, Romania and Estonia recently reported incursions by Russia aircraft over their territories.

Poland reported shooting down multiple drones, while NATO Italian fighter jets escorted three Russian MiG-31 fighter jets away from Estonian territory and Romania scrambled fighter jets to intercept a drone that eventually left the country’s airspace.

The incursions have alarmed European officials, who have issued stern warnings to Russia about further violations. The United Nations Security Council held an emergency meeting Sept. 22 at Estonia’s request, where officials warned Russia that it was risking a confrontation with NATO.

“Our alliance is defensive, but be under no illusion … if we need to confront planes that are operating in NATO airspace without permission, then we will do so,” British Foreign Secretary Yvette Cooper said at the meeting, according to CNN.

Polish Prime Minister Donald Tusk said Sept. 22 that his country would shoot down aircraft when there is a clear incursion on the nation’s territory.

After Trump agreed that NATO nations should shoot down the aircraft, Polish Foreign Minister Radosław Sikorski said in a post on X: "Roger that."

Sikorski told USA TODAY in an interview at the United Nations before Trump's comments that Poland supported Trump's attempt to persaude Putin to end the 3-year-old war. But the approach isn't working, he said, pointing to the incursions and Putin's stepped-up attacks on Kyiv.

"We hope that President Trump will draw the conclusion that personal diplomacy unfortunately hasn’t worked and that he needs to change Putin’s calculus by applying pressure on Russia and by helping Ukraine,” Sikorski said.

Poland borders Ukraine, which was invaded by Russia in 2022.

The confrontation between NATO countries and Russia over the aircraft incursions has the potential to escalate, potentially drawing in the United States.

Article 5 of the NATO treaty states that an attack on one member nation "shall be considered an attack against them all.”

This article has been updated to include additional information,

This article originally appeared on USA TODAY: Trump now says Ukraine can win back its territory, after earlier suggesting concessions

Reporting by Francesca Chambers and Zac Anderson, USA TODAY / USA TODAY

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