Russia said two of its missiles had hit a meeting of Ukrainian military officers on Sunday in the city of Sumy, where Ukraine said Russian strikes had killed 34 people and wounded 117.
Russia's defence ministry accused Ukraine of using civilians as human shields by placing military facilities and holding events involving soldiers in the centre of a densely populated city. There was no immediate response from Kyiv to the "human shield" accusation.
The defence ministry 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.
'Only scoundrels would do this'
President Volodymyr Zelensky on Sunday demanded a tough international response against Moscow over the attack, which came as US President Donald Trump struggles to make progress towards his pledge to rapidly end the war.
"Only scoundrels can act like this, taking the lives of ordinary people," Zelensky said, noting that the attack had come on Palm Sunday when some people were going to church.
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 defence 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.