In a brief, pre-recorded speech at a Stand Up For Ukraine forum organized in Warsaw, Poland, Ukrainian President Volodymyr Zelenskyy called for additional sanctions against Russia and more support. 

"Sanctions must be imposed against all Russian banks. Russian oil embargo must be imposed. The Russian war crimes machine should be denied its capacity to act," he urged. "Vital assistance should be provided to Ukraine. We have asked for weapons. We've asked for financial assistance. We have asked for support to Ukraine and migrants for those 10 million people forced by Russians to flee their homes."

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The leader said he hoped that the courage showed by Ukrainians amidst Russia's continued assault would be shared by everyone across the world and that others should "do everything to force Russia to seek peace" and hold soldiers and their commanders accountable for their crimes against his people. 

Zelenskyy called on the democratic world to aid Ukraine's effort. 

"Ukrainian courage deserves respect. Convince your politicians. Stay with Ukraine. Stay with freedom. Stay with bravery," he said. 

Canadian President Justin Trudeau, Polish President Andrzej Duda and head of the European Commission Ursula von der Leyen also gave short remarks. 

This comes as U.K. Prime Minister Boris Johnson attended a surprise meeting with Zelenskyy in Kyiv on Saturday. 

Johnson's office said the meeting was a "show of solidarity with the Ukrainian people."

It came a day after Johnson pledged another $130 million in high-grade military equipment to Ukraine.

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Following a devastating missile strike on Friday at the Kramatorsk train station – which killed at least 52 people and wounded more than 100 – civilian evacuations moved forward on Saturday before an expected Russian onslaught. 

Russia denied responsibility for the killings and accused Ukraine’s military of firing on the station to try to turn blame for civilian casualties on Moscow. 

Remnants of the rocket had the words "For the children" in Russian painted on it.

Russia had also previously falsely claimed the horrific massacre in the town of Bucha was staged by Ukraine. 

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Ukrainian authorities have warned they expect to find more mass killings once they reach the southern city of Mariupol.

Russia has pulled its troops from norther Ukraine, focusing on Donbas after failing to capture the nation's capital city. 

The Associated Press contributed to this report.