Donald Trump has blamed Volodymyr Zelensky for starting the war with Russia - a day after a massive Russian attack killed 35 people and injured 117 others in Ukraine.

The US president said the Ukrainian leader shared the blame with Russian President Vladimir Putin for "millions of people dead" in the Ukraine war.

"You don't start a war against someone 20 times your size and then hope that people give you some missiles," he told reporters at the White House, also blaming former US President Joe Biden for the conflict.

Trump's comments come after widespread outrage over Russia's attack on the Ukrainian city of Sumy on Sunday, which was the deadliest Russian attack on civilians this year.

Asked about the attack earlier, Trump said it was "terrible" and that he had been told Russia had "made a mistake", but did not elaborate.

"Millions of people dead because of three people," Trump said on Monday. "Let's say Putin number one, let's say Biden who had no idea what the hell he was doing, number two, and Zelensky."

It is estimated that hundreds of thousands, but not "millions", of people have been killed or injured on all sides since Russia launched a full-scale invasion of Ukraine on 24 February 2022.

Questioning Zelensky's competence, Trump remarked that the Ukrainian leader was "always looking to purchase missiles".

"When you start a war, you got to know you can win," the US president said.

Trump has repeatedly clashed with Zelensky since he returned to office this year, and has previously appeared to blame Ukraine for starting the war.

Tensions between the pair have been high ever since their heated confrontation at the White House in February.

During that meeting, Trump accused Zelensky of "gambling with World War Three" and chided him for not starting peace talks with Russia earlier.

By contrast, the US president has made efforts to improve relations with Moscow.

US envoy Steve Witkoff met Putin in St Petersburg on Friday. The Kremlin said talks lasted more than four hours and focused on "aspects of a Ukrainian settlement".

In an interview before Russia's latest attack, Zelensky urged Trump to visit Ukraine before striking a deal with Putin to end the war.

"Please, before any kind of decisions, any kind of forms of negotiations, come to see people, civilians, warriors, hospitals, churches, children destroyed or dead," Zelensky said in an interview for CBS's 60 Minutes programme.

Russia's attack on the city Sumy that killed at least 35 people was "right in the heart of the city on Palm Sunday" Zelensky said at the time.

Moscow said it had fired two Iskander missiles at a meeting of Ukrainian soldiers, killing 60 of them, but did not provide any evidence.

Trump insisted he wanted to "stop the killing" and signalled there would be proposals soon, but did not elaborate.

The conflict goes back more than a decade, to 2014, when Ukraine's pro-Russian president was overthrown. Russia then annexed Crimea and backed insurgents in bloody fighting in eastern Ukraine.