But there was no sign that Trump's words would be matched by a change in US policy, such as a decision to impose the heavy new sanctions on Moscow sought by Ukrainian President Volodymyr Zelenskiy as he traveled to New York this week.

"Putin and Russia are in BIG economic trouble, and this is the time for Ukraine to act," Trump wrote on Truth Social, shortly after meeting Zelenskiy on the sidelines of the UN General Assembly in New York.

"After seeing the economic trouble [the war] is causing Russia, 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," he said.

That would ostensibly require Kyiv to expel Russian forces from 20 percent of its territory, including the Crimean peninsula Moscow has held since 2014, in what would be an extraordinary reversal.

Trump has previously suggested Kyiv should consider giving up territory in order to make peace, fueling Ukrainian fears of behind-the-scenes talks for a deal that would seek to recognize its occupied lands as legally Russian.

Europe's foreign policy chief, Kaja Kallas, praised Trump's statements, saying: "These have been very strong statements that we haven't heard before in such formats, so it is really good that we are in the same understanding now."

Good, constructive meeting

The US president's tone was in stark contrast to his red-carpet treatment for Russian President Vladimir Putin at a summit in Alaska last month, part of an ostensible push to expedite an end to Russia's war in Ukraine.

Zelenskiy told reporters at a briefing that he had a "good, constructive" meeting with Trump, declining to go into detail, while praising Trump's statement on Truth Social as a "big shift."

Zelenskiy later told Fox News that he thought the positions of the Ukrainian and US teams were "closer than any time before," and that he thought Trump's position had changed.

The US statement criticized Russia, saying it had been fighting "aimlessly" in a war that a "real military power" would have won in less than a week. That, Trump added, made Russia look very much like a "paper tiger".

Nonetheless, the only firm commitment from Trump on Truth Social was to "continue to supply weapons to NATO for NATO to do what they want with them," an apparent reference to a new mechanism allowing European countries to buy US weapons for Ukraine.

US Secretary of State Marco Rubio's remarks to the UN Security Council suggested the United States had not given up hope of a peaceful resolution.

"This war needs to end. But if it does not, if there is no path to peace in the short term, then the United States and President Donald J. Trump will take the steps necessary to impose costs for continued aggression."

Zelenskiy has been urging the United States to ramp up sanctions pressure on Russia to coerce it into entering negotiations to end the war launched in February 2022, a call he repeated at the United Nations.

Addressing the General Assembly earlier, Trump said he was ready to impose strong economic measures if Russia did not end its war, but that allies would have to do the same. He also derided some European powers for continuing to buy Russian oil.

Zelenskiy said he and Trump discussed Russia's stuttering economy and "there was an understanding" that Trump would be ready to provide security guarantees to Ukraine when the war ended.

He said that Trump had the power to prove a "game-changer" for Ukraine in the war. Zelenskiy noted China retained influence over Russia, though he added he had seen no sign from Beijing that it wanted the war over.

Speaking to reporters, Russia's Deputy UN Ambassador Dmitry Polyanskiy played down the significance of Trump's statement on Truth Social.

"Don't get so excited about every tweet," said Polyanskiy.

Former Democratic US Representative Tom Malinowski said the Trump statement was "an amazing 180-degree turn, which might not last long".

Malinowski, also a former assistant US secretary of state, added on X: "But Putin will have only one question back. What more is Trump actually going to do to help Ukraine win? If nothing, then it's just words.