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.
Image: Donald Trump and Volodymyr Zelenskyy at the United Nations. Pic: Reuters
Writing on Truth Social, the US president said: “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.
“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.”
Mr Trump wrote that 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 added: “This is not distinguishing Russia. In fact, it is very much making them look like ‘a paper tiger’.”
Mr Trump’s tone contrasted greatly with his red-carpet treatment for the Russian president at a summit in Alaska last month, part of a push to expedite an end to the war in Ukraine.
The US president has previously suggested Ukraine would never be able to reclaim all the territory Russia has occupied since seizing the Crimean Peninsula in 2014.
Mr Zelenskyy later said he was surprised by Mr Trump’s comments, telling Fox News’ Special Report he has a better relationship with the US president than before.
He said the comments were a very positive signal Mr Trump and the US will be with Ukraine until the end of the war.
Please use Chrome browser for a more accessible video player
4:11
Rigby to Trump: Was Putin’s Alaska invite a mistake?
Earlier on Tuesday, while talking to Mr Zelenskyy at the United Nations, Mr Trump 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”.
Please use Chrome browser for a more accessible video player
2:42
Trump attacks UN and London mayor
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”.
Kyiv 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”.
In a joint statement following the president’s comments, G7 foreign ministers said discussions were ongoing about additional economic sanctions on Russia.
Mr Trump also suggested the Russian people are not aware of “what is really going on with this war”.
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.
Image: The situation in Ukraine on 19 September
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 Italian NATO jets escorted them away.
The week before, about 20 Russian drones entered Polish airspace, prompting NATO jets to shoot some of them down.