By Yurii Kovalenko and Dan Peleschuk

KYIV (Reuters) - Ukrainians braced on Thursday for a hopeful but perilous phase of their war with Russia after U.S. President Donald Trump's phone call with Vladimir Putin stirred fears his move to swiftly end the fighting could cost Ukraine dearly.

Trump's call to the Russian president - the first by a U.S. leader since Moscow's February 2022 invasion - followed remarks by his defence chief that cast doubt over the full restoration of Ukraine's borders and its aspiration to join the NATO alliance.

"It truly looks as if they want to surrender Ukraine, because I don't see any benefits for our country of these negotiations or Trump's rhetoric," said Kyiv resident Myroslava Lesko, 23, standing near a sea of flags downtown honouring fallen troops.

Ukraine has tried to urgently forge close ties with the new Trump administration, offering a minerals deal to win U.S. favour, while Russian forces, already occupying a fifth of Ukraine, keep up battlefield pressure.

Ukraine wants the White House to back any peace talks with security guarantees that would stop Russia invading again.

The Trump call with Putin on Wednesday and remarks by Pete Hegseth, who said the U.S. would not be sending in troops as peacekeepers, have shocked some of Kyiv's allies who believe Washington is sacrificing negotiating leverage.

Trump spoke with Ukrainian President Volodymyr Zelenskiy after his call with Putin.

In interviews with Reuters, Kyiv residents reacted to the prospect of fast-track peace talks with a mix of dismay and cautious hope.

"Trump is a strong-willed person," said 60-year-old Hryhoriy Buhoyets. "He can decide on whatever he wants, and regarding Ukraine, I think he has some plans."

Maksym Zhorin, deputy commander of the Third Assault Brigade fighting in the war, wrote on the Telegram app that he had not expected Ukraine to receive quick NATO membership anyway.

"NATO is not ready for war with the Russian Federation. This is evident from their reaction to events and behaviour," he said.

Tymofiy Mylovanov, president of the Kyiv School of Economics, said it looked like the Trump administration's views were in fact largely in keeping with those of the previous Joe Biden administration.

Some Ukrainians have expressed frustration that the U.S. under Biden, despite providing billions in military aid, had not done enough to tip the war in Kyiv's favour.

"The difference between Biden and Trump is that Trump says out loud what Biden was thinking and doing about Ukraine," he wrote on X.

'FASTEN YOUR SEATBELTS'

Ukrainian Foreign Minister Andrii Sybiha told Le Monde newspaper in an interview released on Thursday that Ukraine and Europe should not be excluded from future peace talks.

But some officials have urged Ukrainians - many of whom have lost loved ones in fighting that has killed tens of thousands and ravaged vast swathes of the country - not to jump to any conclusions after the remarks.

"Ukraine doesn't exist because someone had a good conversation on the phone with someone. We survived because we showed the world that we can fight," said opposition lawmaker Oleksii Honcharenko.

Some officials, who have been lavishing praise on Trump in their effort to build relations, have tried to quickly extinguish what one of them described as "too many unnecessary rumours and conspiracy theories".

"A difficult process of fighting for Ukraine lies ahead, and we will go through it in unity," said Daria Zarivna, an adviser to Zelenskiy's chief of staff.

Olena Chiupika, 38, a manager in Kyiv, said Ukraine had already proven successful at securing foreign support that had once seemed unrealistic, citing the F-16 fighter jets provided by allies .

"I'm hoping for the best," she said. "I would like to, because the mood is not great."

The next diplomatic test for Ukraine is expected at the Munich Security Conference when Zelenskiy is likely to meet U.S. Vice President JD Vance.

Hanna Maliar, a former deputy defence minister, appeared to acknowledge the uncertainty of the moment.

"Fasten your seatbelts. We're taking off," she said shortly after news broke of Trump's call with Putin. "A fascinating journey awaits us. The final destination is so far unknown."

(Additional reporting by Yuliia Dysa and Pavel Polityuk; Writing by Dan Peleschuk; editing by Tom Balmforth and Andrew Cawthorne)