Ben Wallace vowed to "do everything" to ensure Vladimir Putin fails in Ukraine and predicted the war would backfire for the Russian president, perhaps getting more nations to join the Nato defence alliance.
Ukrainian President Volodymyr Zelensky said at least 30 people were killed on the attack on the crowded Kramatorsk station that was an evacuation point for civilians.
A further 100 people were injured, Zelensky said as he blamed Russia for "an evil without limits", while the Kremlin denied targeting the station.
Foreign Secretary Liz Truss said she was "appalled" by the reports, adding: "The targeting of civilians is a war crime. We will hold Russia and Putin to account."
Wallace, visiting Romania for talks, said the strike was a repeat of the Russian president and his generals targeting civilians.
Speaking in Constanta, he said: "Not very far away this morning in a place called Kramatorsk, what appears to be Russian missiles struck civilian people queuing for trains to seek a safer place from the war.
"The striking of civilians and critical infrastructure is a war crime. These were precision missiles aimed at people trying to seek humanitarian shelter."
Pressed if he thought the station attack was a war crime, Wallace said evidence would be collected to prove the answer, but added: "The circumstantial evidence absolutely points to a range of war crimes being committed by Russia."
Wallace insisted Mr Putin's war would backfire.
"There will be more Ukraine after this, there will be more forces on the border with Nato, there may be more Nato members as a result of Putin's actions," he said.
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