Washington DC - The US Defense Department says Russian forces were responsible for the deadly missile attack on a train station in the Ukrainian city of Kramatorsk that killed at least 50 people, and said Russian troops are gathering near Kharkiv in the ongoing war in Ukraine.

Russia's official denials of the attack at the Kramatorsk railway station are "unconvincing," Pentagon spokesperson John Kirby said. "Our assessment is that this was a Russian strike and that they used a short-range ballistic missile to conduct it."

Referring to the death toll, he said the attack was yet another example of Russian "brutality" and "carelessness" concerning civilian casualties.

The European Union has also now blamed Russia for the missile attack on Ukraine, calling it a war crime. Ukrainian President Volodymyr Zelensky said on Friday he expects a decisive reaction from the international community, reiterating calls for a full energy embargo on Moscow and for Russian banks to be fully excluded from international finance.

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Meanwhile, thousands of Russian forces have gathered near the edge of Kharkiv, according to a senior US defense official, a city in the north-east of Ukraine, close to the Russian border.

The number of tactical units near the Russian border city of Belgorod has risen from 30 to 40, the official said Friday. He did not give an exact number of additional troops but said these types of battalion are typically made up of 600 to 1,000 soldiers.

The Russian military is gathering its forces there in order to concentrate its troops on taking over the eastern Ukrainian Donbass region, the official said. He added there were indications Russia hoped to mobilize more than 60,000 soldiers.

Because the Donbass is familiar territory for both Ukraine and Russia's armies after years of conflict there, he warned fighting would be intense.

"This will be a knife fight. This could be very bloody and very ugly," the official said.

Pentagon spokesman John Kirby earlier said there were reports that tens of thousands of reservists were being mobilized to reinforce Russia's units on the Donbass border after losses in northern Ukraine.