The US president says Russia has been « fighting aimlessly » for three-and-a-half years and would have won quickly if it was a « real military power ».
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 correspondent Mark Stone 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 ».
