Donald Trump has described Russia as a “paper tiger” and said Ukraine could get its territory back in its “original form”.
Volodymyr Zelenskyy, the Ukrainian president, told Sky News it was a “big shift” from his US counterpart.
Mr Trump, speaking to French president Emmanuel Macron at the United Nations in New York, said his relationship with Vladimir Putin had turned out to be meaningless.
Writing on Truth Social, the US president said he had gained a greater understanding of the “economic trouble” the war was causing Moscow.
He said Russia had been “fighting aimlessly” for three-and-a-half years and had it been a “real military power” it would have defeated Ukraine in less than a week.
Mr Trump commented: “This is not distinguishing Russia. In fact, it is very much making them look like “a paper tiger”.
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Earlier, while talking to Mr Zelenskyy at the United Nations, he was asked by reporters whether he thought NATO should shoot down Russian planes if they entered NATO airspace.
“Yes, I do,” Mr Trump replied.
Asked whether the US would support NATO in shooting down Russian aircraft, Mr Trump said it depended on the circumstances.
On Truth Social, he said the US would continue to supply weapons to NATO and it was for the military alliance to “do what they want with them”.
Ukraine, he said, with the “support of the European Union”, is in a position to “fight and win all of Ukraine back in its original form”.
Ukraine would need the “financial support of Europe and, in particular, NATO”, he said
But, given those caveats, he said the “original borders from where this war started is very much an option”.
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The Russian people are not aware of “what is really going on with this war”, Mr Trump suggested.
He added: “Most of their money is being spent on fighting Ukraine. Putin and Russia are in big economic trouble and this is the time for Ukraine to act.”
Ukraine has lost large areas of land in the east of the country. In the Donetsk region, Russia now controls about 70% of the territory. Kyiv’s forces have been pushed back to a string of four cities analysts have dubbed the “fortress belt”.
Moscow has partly annexed three other regions, too – Luhansk in the east, Zaporizhzhia and Kherson
further west.
Meanwhile, Russia appears to be provoking its neighbours to the west. Last week, Estonia said three Russian MiG-31 fighter jets violated its airspace for 12 minutes before NATO Italian jets escorted them away.
The week before, about 20 Russian drones entered Polish airspace, prompting NATO jets to shoot some of them down.
NATO has now beefed up its defence of Europe’s eastern flank.