For days, Ukraine's president has warned that Russian forces may attempt "something particularly cruel" this week.
He told the UN Security Council that the deadly attack occurred in the town of Chaplyne in the central Dnipropetrovsk region.
Ukraine had been bracing for particularly heavy attacks in the run-up to the national holiday commemorating Ukraine's declaration of independence from the Soviet Union in 1991, which fell this year on the six-month mark of the Russian invasion in 2022.
It comes as Prime Minister Boris Johnson returns to the Ukrainian capital of Kyiv, where he was awarded the Order of Liberty, Ukraine's highest award bestowed on foreign nationals.
Mr Johnson, on his final trip to the country before leaving office, announced an additional £54 million in military aid, including drones to target Russian forces.
"What happens in Ukraine is matters to us all," Mr Johnson posted on Twitter.
"That is why I am here today in Kyiv." That is why the United Kingdom will continue to support our Ukrainian allies.
"I am confident that Ukraine can and will win this war."
The most recent aid package for Ukraine includes unmanned surveillance and missile systems, as well as 2,000 cutting-edge drones and anti-tank loitering munitions.
850 hand-launched Black Hornet micro-drones are also included, which can provide troops with live feeds and still images.
During his visit to Ukraine, Mr Johnson also suggested that high energy bills are something that UK households must bear in order to resist Vladimir Putin.
"If we're paying in our energy bills for Vladimir Putin's evils, the people of Ukraine are paying in their blood," he said.