The Kremlin says the Putin-Trump talks could last at least six to seven hours.
Ukrainian President Volodymyr Zelenskiy, who was not invited to the talks, and his European allies fear Trump might sell out Ukraine by essentially freezing the conflict and recognising, if only informally, Russian control over one fifth of Ukraine.
Trump sought to assuage such concerns as he boarded Air Force One, saying he would let Ukraine decide on any possible territorial swaps.
"I'm not here to negotiate for Ukraine, I'm here to get them at a table," he said.
Zelenskiy said Russia was continuing to attack Ukraine ahead of a summit.
"On the day of the negotiations, they also kill people. And that says a lot," Zelenskiy said on the Telegram messaging app.
Earlier, Ukrainian regional officials said Russia had launched a ballistic missile into Dnipropetrovsk region in eastern Ukraine, killing one person and wounding at least one other.
Both the US and Russian presidents, due to meet at a Cold War-era air force base in Alaska's largest city, are seeking wins from their first face-to-face talks since Trump returned to the White House.
Trump, who casts the war as a "bloodbath", is pressing for a truce in the 3.5-year-old war that would bolster his credentials as a global peacemaker worthy of the Nobel Peace Prize.
For Putin, the summit is already a big win as he can use it to say that years of Western attempts to isolate Russia have unravelled and that Moscow has retaken its rightful place at the top table of international diplomacy.
In a symbolic gesture en route for Alaska, Putin laid flowers on Friday at a memorial in Russia's Far East commemorating US-Soviet cooperation in World War Two.
The Kremlin said Putin would arrive in Alaska at 11am (5am Saturday AEST) and that Trump would meet the Russian leader at his plane.
Trump, who once said he would end Russia's war in Ukraine within 24 hours, conceded on Thursday it had proven a tougher nut to crack than he thought.
He said that if talks went well, quickly arranging a second three-way summit with Zelenskiy would be even more important than his encounter with Putin.
One source acquainted with Kremlin thinking said there were signs Moscow could be ready to strike a compromise on Ukraine, given that Putin understood Russia's economic vulnerability and costs of continuing the war.
"He is a smart guy, been doing it for a long time but so have I... We get along, there's a good respect level on both sides," Trump said, describing Putin.
Reuters has previously reported that Putin might be willing to freeze the conflict along the front lines, provided there was a legally binding pledge not to enlarge NATO eastwards and to lift some Western sanctions.
Russia, whose war economy is showing signs of strain, is vulnerable to further US sanctions.
Trump has threatened tariffs on buyers of Russian crude, primarily China and India.
"For Putin, economic problems are secondary to goals, but he understands our vulnerability and costs," the Russian source said.
On the eve of the summit, Putin held out the prospect of something else he knows Trump wants, a new nuclear arms control agreement to replace the last surviving one, which is due to expire in February next year.
Ukrainians who spoke to Reuters in central Kyiv on Friday were not optimistic about the Alaska summit.
"Nothing good will happen there, because war is war, it will not end. The territories - we're not going to give anything to anyone," said Tetiana Harkavenko, a 65-year-old cleaner.
