Russia said two of its missiles hit a meeting of Ukrainian military officers on Sunday in the city of Sumy, after Ukraine called the strike a deliberate attack on civilians.
Local prosecutors in the northern Ukrainian city said the death toll rose to 35 on Monday, with 117 people wounded. In a statement, Russia's defense ministry accused Ukraine of using civilians as human shields by placing military facilities and holding events involving soldiers in the center of a densely populated city.
There was no immediate response from Kyiv to the "human shield" accusation.
Foreign Minister Andrii Sybiha said Russian attacks on Sumy and the city of Kryvyi Rih showed Russian President Vladimir Putin was seeking a continuation of war, not an end to it. The Kremlin says Russia is willing to seek a lasting peace that addresses what it calls the root causes of the conflict.
The Russian statement said its forces had fired "two Iskander-M tactical missiles at the meeting venue" of what it called an operational tactical group of Ukraine's armed forces.
It said that more than 60 Ukrainian soldiers were killed in the strike.
Russian Foreign Minister Sergei Lavrov told the Kommersant newspaper that Ukrainian military leaders had been meeting in Sumy with "Western colleagues", but did not identify any Western participants or provide evidence to support the allegation.
Reuters has contacted the foreign ministry for comment and received no immediate response.
President Volodymyr Zelenskiy on Sunday demanded a tough international response against Moscow over the attack, which came as US President Donald Trump struggles to make progress towards fulfilling his pledge to rapidly end the war.
"Only scoundrels can act like this, taking the lives of ordinary people," Zelenskiy said, noting that the attack took place on Palm Sunday, the Sunday before Easter when many people go to church.
Zelenskiy's chief of staff Andriy Yermak said Russia was trying to "kill as many civilians as possible". Sybiha said Kyiv was "sharing detailed information about this war crime with all of our partners and international institutions".
The leaders of Britain, Germany and Italy condemned the attack. Trump, when asked about the Russian strike, said that it was terrible.
"And I was told they made a mistake," he said without elaborating further. "But I think it's a horrible thing."
Russian presidential spokesman Dmitry Peskov was asked at his daily briefing how the Kremlin viewed Trump's comment and whether the strike had been conducted in error.
He replied that the Kremlin did not comment on the course of the war, and this was a matter for the defense ministry.
"I can only repeat and remind you of the repeated statements of both our president and our military representatives that our military strikes exclusively at military and military-adjacent targets," he said.
A United Nations monitoring mission said in February that at least 12,654 Ukrainian civilians had been killed in the first three years of the war and 29,392 had been wounded.
French President Emmanuel Macron said the attack on Sumy highlighted the urgent need to impose a ceasefire on Russia, and Polish Foreign Minister Radoslaw Sikorski said Putin was mocking the goodwill of Trump and his administration.
