US President Donald Trump has said Kyiv can “win all of Ukraine back in its original form”, marking a major shift in his position on the war with Russia.

In a post on his Truth Social platform, he said Ukraine could get back “the original borders from where this war started” with the support of Europe and Nato, due to pressures on Russia’s economy.

His comments came after talks with Ukrainian President Volodymyr Zelensky, held after Trump had addressed the UN General Assembly in New York on Tuesday.

Trump has repeatedly expressed his desire to end the war, but has previously warned that process would likely involve Ukraine giving up some territory, an outcome Zelensky has consistently rejected.

In his post, Trump added Ukraine could “maybe even go further than that”, but did not specify what he was referring to.

He also made no reference to Crimea, which was invaded and annexed by Russia in 2014. Russia launched its full-scale invasion of Ukraine in 2022.

Trump said his position had changed “after getting to know and fully understand the Ukraine/Russia military and economic situation”.

“Putin and Russia are in BIG Economic trouble, and this is the time for Ukraine to act,” he added, labelling Russia as a “paper tiger”.

In response, Russian President Vladimir Putin’s spokesman, Dmitry Peskov, said “Russia is in no way a tiger”.

“It’s more associated with a bear and there is no such thing as a paper bear,” he told Russia’s RBK Radio.

He added that the Kremlin leader “values the US president’s efforts to seek a resolution to the Ukraine conflict”.

Zelensky hailed the “big shift” in Trump’s position, and speaking to reporters in the UN building, said he understood the US was willing to give Ukraine security guarantees “after the war is finished”.

Pressed on what this would look like, he added: “I don’t want to lie, we don’t have specific details,” but broached the possibility of more weapons, air defences and drones.

Speaking later on Fox News, Zelensky said that Trump’s Truth Social post about Ukraine’s positioning surprised him but he took it as a “positive signal” that Trump and the US “will be with us to the end of the war”.

“I think the fact that Putin was lying to President Trump so many times also made a difference between us,” he told Fox host Bret Baier.

Earlier on Tuesday, following his speech to the UN, Trump also said Nato nations should shoot down Russian planes breaching their airspace, following a series of recent incursions by Russian fighter jets and drones.

Poland’s foreign minister, Radoslaw Sikorski, responded on social media with a brief message of approval: “Roger that.”

Last week, Estonia and Poland requested a consultation with other Nato members after Russia violated its airspace in separate incidents. Romania, another Nato member, also said Russian drones breached its airspace.

After meeting on Tuesday, Nato issued a statement condemning Russia’s actions and warned that it would use “all necessary military and non-military tools” to defend itself.

“Russia bears full responsibility for these actions, which are escalatory, risk miscalculation and endanger lives. They must stop,” it said.

The alliance added that Moscow’s actions were part of a “pattern of increasingly irresponsible” behaviour.

Nato’s secretary general Mark Rutte said: “We are a defensive alliance, yes, but we are not naive, so we see what is happening.”

Speaking at the UN, Poland’s president Karol Nawrocki echoed those comments, saying his country was prepared to “defend its territory” and “react adequately”.

“Polish people, as well as countries of Central and Eastern Europe, will not be scared of Russian drones,” he said.

Russia denied violating Estonia’s airspace, while it insisted the Polish incursion was not deliberate and did not comment on the Romania incident.

Asked if the US would support its Nato allies if they shot down Russian aircraft, Trump said it “depends on the circumstance” and praised the military alliance for increasing defence spending.

“Nato has stepped up,” he said, referring to an agreement by leaders to ramp up defence spending to 5% of their countries’ economic output by 2035.

In his speech hours earlier, Trump criticised some Nato members for not ceasing the purchase of Russian energy, saying they were “funding a war against themselves”.

Trump also met European Commission President Ursula von der Leyen at the UN on Tuesday, and she made clear that the European Union was tightening the pressure on Russian oil and gas exports. “By 2027, Europe will have turned the page on Russian fossil fuels for good,” she posted on X.

Tuesday’s Truth Social post represents an about-face after Trump spent most of the year insisting that Ukraine’s situation was dire.

In February, Trump told Zelensky during their fiery Oval Office exchange that he did “not have the cards right now” to prevail against a larger, more populous nation in a war of attrition.

Before talks with Russian President Vladimir Putin in Alaska in August, Trump said he would try to get some territory back for Ukraine but warned there would be “some swapping, changes in land”.

There were reports he was planning to press Zelensky to surrender the entirety of the eastern Ukrainian regions of Donetsk and Luhansk in return for Russia freezing the rest of the front line – a proposal put forward by Putin in Alaska.

Trump has also repeatedly threatened to impose tougher measures on Russia, but has so far failed to take any action when the Kremlin ignored his deadlines and threats of sanctions.

Unpredictability has long been one of the US president’s foreign policy trademarks, and perhaps this latest move is an attempt to shake up peace negotiations that have been stagnant for more than a month after Trump hosted Russian President Vladimir Putin at a summit in Alaska.

The most notable portion of Trump’s post may be the way it ended – with an assurance the US would continue to sell arms to Nato that it could then pass along to Ukraine.

It is not the seemingly near open-ended commitment to the war effort that the Biden administration provided, but it is more than Trump seemed interested in offering at times this year.