As regional authorities scrambled to evacuate the vulnerable, European Union leaders visited Kyiv to offer President Volodymyr Zelenskiy support and expedite Ukraine's path toward EU membership.
Zelenskiy called the strike in Kramatorsk in the eastern region of Donetsk a deliberate attack on civilians. The town's mayor estimated about 4000 people were gathered there at the time.
Regional governor Pavlo Kyrylenko said the station was hit by a Tochka U short-range ballistic missile containing cluster munitions, which explode in mid-air, spraying small lethal bomblets over a wider area.
Reuters was unable to verify what happened in Kramatorsk.
Cluster munitions are banned under a 2008 convention. Russia has not signed it but has previously denied using such armaments in Ukraine.
In Washington, a senior defence official said the United States was "not buying the denial by the Russians that they weren't responsible", and believed Kyrylenko correctly identified the type of missile used in the attack.
The Russian defence ministry was quoted by RIA news agency as saying the missiles said to have struck the station were used only by Ukraine's military and that Russia's armed forces had no targets assigned in Kramatorsk on Friday.
Zelenskiy told Finland's parliament in a video address that no Ukrainian troops were at the station.
"We expect a firm global response to this war crime," he said later in a speech posted online.
Moscow has denied targeting civilians since President Vladimir Putin ordered the invasion of Ukraine on February 24 in what Russia calls a "special military operation" to demilitarise and "denazify" its neighbour.
Kyiv and Western supporters call that a pretext for an unprovoked invasion that has displaced a quarter of the population and killed or injured thousands.
Ukrainian officials now expect an attempt by Russian forces to gain full control of Donetsk and neighbouring Luhansk, both partly held by Moscow-backed separatists since 2014.
The Kremlin on Friday said the "special operation" could end in the "foreseeable future" with its aims being achieved through work by the Russian military and peace negotiators..
The White House said it would support attempts to investigate the attack in Kramatorsk.
British Prime Minister Boris Johnson said it showed "the depths to which Putin's vaunted army has sunk".
At least 52 people have died in the incident, Pavlo Kirilenko, head of the Donetsk regional military administration, said in an online post.
The wreckage of the missile bore the words "(This is) for the children" on its side. Russia has for years accused Ukraine of killing civilians including children with strikes in separatist-held eastern Ukraine.
As Russia concentrates on the east, Ukrainian forces there said late on Friday that they had repelled seven Russian attacks, destroying nine tanks, seven other armoured vehicles and two helicopters. Reuters could not independently verify that.
Following a partial Russian pullback near Kyiv, a forensics team on Friday began exhuming a mass grave in the town of Bucha. Authorities say hundreds of dead civilians have been found there.
Russia has called allegations that its forces executed civilians in Bucha a "monstrous forgery" aimed at denigrating its army and justifying more sanctions.
Russian forces have failed to take any major cities so far, confronted by unexpectedly strong resistance and dogged by what Western intelligence officials say have been logistical, supply and morale problems.
Kyiv wants deliveries of heavier armaments and on Thursday secured a new commitment from the NATO alliance to supply a wide range of weapons.
Residents of areas north of Kyiv were meanwhile still coming to terms with a month-long occupation.
In the village of Yahidne, residents recounted how more than 300 people were trapped for weeks in a school basement, with names of those who did not survive or were killed by soldiers scrawled on the wall.
Reuters was not able to verify independently the accounts. Reporters saw one freshly dug grave and two bodies wrapped in white plastic sheets.
