US President Donald Trump said on Tuesday that Ukraine could win back all of its territory from Russia – and even go further – in a major pivot after meeting President Volodymyr Zelensky.

The astonishing turnaround came shortly after Trump also called for Nato countries to shoot down any Russian jets that violate their airspace.

"I think Ukraine, with the support of the European Union, is in a position to fight and WIN all of Ukraine back in its original form," Trump said on his Truth Social network after his talks with Zelensky.

Trump also said Russia was "fighting aimlessly" after three years of war, in an apparent change of heart just over a month after he met Russian President Vladimir Putin in Alaska.

The US president has previously shown lukewarm support for Zelensky, with whom he had a huge televised Oval Office bust-up in February during which he told the Ukrainian "you don't have the cards" to win.

But in his social media post on Tuesday, Trump dismissed Russia as a "paper tiger," saying that "Putin and Russia are in BIG Economic trouble, and this is the time for Ukraine to act."

"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 wrote.

He added that as Russia's economy gets worse "Ukraine would be able to take back their Country in its original form and, who knows, maybe even go further than that!"

Zelensky hailed the change in Trump's position.

"This post of Trump is a big shift," Zelensky said in a press conference.

He also said there was an understanding that Trump was ready to provide security guarantees for Ukraine after the war ends, adding that Trump could be a "game-changer" for Ukraine.

"All of us, we understand that President Trump is ready to give Ukraine security guarantees after this war will finish," Zelensky said. (Agencies)