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.

Russian President Vladimir Putin's spokesman, Dmitry Peskov, suggested the reason for Trump's change of tone was that he had just talked to the Ukrainian president. The US president, he told reporters, had made the comments "apparently under the influence of the vision put forward by Zelensky… this vision is in absolute contrast with our understanding of the current state of affairs".

"The thesis that Ukraine can fight something back, we believe it erroneous. The situation on the front line speaks for itself." – BBC TRUE NEWS

Trump explained that 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".

The Kremlin spokesman responded by insisting "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 also denied Russia was in economic trouble, but conceded there were "points of tension" in some sectors linked to global sanctions on Russia.

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.