Donald Trump has said Volodymyr Zelensky shares the blame with Russian President Vladimir Putin for "millions of people dead" in the Ukraine war.
The US president was responding to reporters' questions during a meeting with El Salvador's leader at the White House.
"When you start a war you got to know you can win," he said, also blaming former US President Joe Biden for the conflict.
Trump's comments come after widespread outrage over the deadliest Russian attack on civilians this year, when missiles struck the north-east Ukrainian city of Sumy on Sunday.
Trump said earlier the Russian attack had been a "mistake".
"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".
"You don't start a war against someone 20 times your size and then hope that people give you some missiles," Trump said.
Tensions between Trump and the Ukrainian leader have been high ever since their heated confrontation at the White House in February.
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 killed at least 35 people and injured 117 others.
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.
