At least 39 people were killed and dozens wounded when rockets hit a train station in the city of Kramatorsk in eastern Ukraine.
Donetsk regional governor said thousands of people trying to flee Russian attacks were at the station during the attack.
Russia invaded Ukraine in February and the war has defied a series of peace talks.
According to BBC, Russia has denied involvement in the strike, with a Kremlin spokesman saying it had no scheduled missions in the area
Russia is believed to be stepping up its offensive in the Donbas region of eastern Ukraine as it withdraws from the north.
Cities in Donbas came under heavy Russian attack overnight, with residents sheltering in basements and Severodonetsk hit by rocket fire.
However, UK PM Boris Johnson is meeting German Chancellor Olaf Scholz for talks and will give a joint news conference later.
European Commission Chief Ursula Von der Leyen has gone to Kyiv for talks with President Zelensky.
Residents in Yahidne, near Chernihiv, told our source that Russian troops held 130 people in a basement for four weeks.
