This week's Alaska summit between US President Donald Trump and Russian President Vladimir Putin "is a listening exercise for the president", the White House says, tempering expectations for a quick Russia-Ukraine ceasefire deal.

Small bands of Russian soldiers have thrust deeper into eastern Ukraine ahead of the summit, which European leaders fear could end in peace terms imposed on an unlawfully shrunken Ukraine.

In one of the most extensive incursions so far this year, Russian troops advanced near the coal-mining town of Dobropillia, part of Putin's campaign to take full control of Ukraine's Donetsk region. Ukraine's military dispatched reserve troops, saying they were in difficult combat against Russian soldiers.

In the first US-Russia summit since 2021, Putin and Trump will meet on Friday at Elmendorf Air Force Base in Anchorage, Alaska, two White House officials said.

Zelensky tells youth forum Putin and Trump cannot agree on anything without Ukraine. – AP

Trump has said any peace deal would involve "some swapping of territories to the betterment of both" Russia and Ukraine, which has up to now depended on the US as its main arms supplier.

But because all the areas being contested lie within Ukraine, President Volodymyr Zelensky and his European Union allies fear that he will face pressure to give up far more than Russia does.

Trump's administration tempered expectations on Tuesday for major progress toward a ceasefire, calling his meeting on Friday with Putin in Alaska a "listening exercise".

Along that line, US Secretary of State Marco Rubio said the president wanted to size up Putin directly.

"The president feels like, 'look, I’ve got to look at this guy across the table – I need to see him face to face, I need to hear him one on one, I need to make an assessment by looking at him'," Rubio said.

Zelensky and most of his European counterparts have said a lasting peace cannot be secured without Ukraine at the negotiating table, and a deal must comply with international law, Ukraine's sovereignty and its territorial integrity.

They will hold a virtual meeting with Trump on Wednesday to underscore those concerns before the Putin summit, the first US-Russia summit since 2021.

Ukrainian air-defence unit tries to halt Russian assault in Donetsk region. – AP

"Substantive and productive talks about us without us will not work," Zelensky said in a TV interview.

"They are possible, but they will not be accepted in practice. Just as I cannot say anything about another state or make decisions for it.”

Zelensky has said Russia must agree to a ceasefire before territorial issues are discussed. He would reject any Russian proposal that Ukraine pull its troops from the eastern Donbas region and cede its defensive lines.

Asked why Zelensky was not joining the US and Russian leaders at the Alaska summit, White House spokeswoman Karoline Leavitt said he bilateral meeting had been proposed by Putin, and that Trump accepted to get a "better understanding" of how to end the war.

Firefighters at the site of a Russian strike in Zaporizhzhia, Ukraine. – AP

"Only one party that's involved in this war is going to be present, and so this is for the president to go and to get, again, a more firm and better understanding of how we can hopefully bring this war to an end," she said.

"This is a listening exercise for the president."

Trump will meet one-on-one with Putin during the talks, which will take place in Anchorage, Alaska, the White House said. He may, in the future, also visit Russia.

"Perhaps there are plans in the future to travel to Russia," said Leavitt.

Trump is open to a trilateral meeting with Putin and Zelensky later, Leavitt said.

Ukraine and its European allies fear that Trump, keen to claim credit for making peace and seal new business deals with Russia's government, will end up rewarding Putin for 11 years of efforts to seize Ukrainian territory, the last three in open warfare.

T-shirts with images of Vladimir Putin and Donald Trump on sale in Moscow. – Reuters

European leaders have said Ukraine must be capable of defending itself if peace and security are to be guaranteed on the continent, and that they are ready to contribute further.

"Ukraine cannot lose this war and nobody has the right to pressure Ukraine into making territorial or other concessions, or making decisions that smack of capitulation," Polish Prime Minister Donald Tusk said at a government meeting.

"I hope we can convince President Trump about the European position."

Zelensky has said he and European leaders "all support President Trump's determination".

Hungarian Prime Minister Viktor Orban, Putin's principal ally in Europe, was the only leader not to join the EU's statement of unity. He mocked his counterparts as "sidelined" and said Russia had already defeated Ukraine.

"The Ukrainians have lost the war. Russia has won this war," Orban said during a YouTube channel interview.

Trump had been recently hardening his stance towards Russia, agreeing to send more US weapons to Ukraine and threatening hefty trade tariffs on buyers of Russian oil in an ultimatum that has now lapsed.