Ukrainian President Volodymyr Zelenskyy warned of continued devastation on par with what has been seen in Mariupol and Volnovakha amid heavy fighting in the eastern part of the country.

"Russia wants to destroy every city in Donbas, 'every' is not an exaggeration," he said in his latest national address.

"This is probably the fastest example of the complete degradation of any state -- the path that the Russian state has covered in 107 days," the president continued.

In recent days, Russian forces have encircled Severodonetsk, the largest city still held by Ukrainian troops in contested Donbas. Zelenskky has said the battle there could determine the fate of eastern Ukraine.

"The Ukrainian troops are doing everything to stop the offensive of the occupiers. As much as possible. As much as the heavy weapons, modern artillery -- all that we have asked and continue to ask our partners for -- allow them to," the president said Friday.

Up to 100 Ukrainian soldiers are killed and another 500 are injured each day, according to Ukrainian Defense Minister Oleksiy Reznikov.

Reznikov said in a Facebook post Thursday that Russia has "many more means in store to devour human lives in a bid to satisfy its imperial ego."

"That is why we emphasize: Ukraine desperately needs heavy weapons, and very fast," Reznikov said.

Among the weapons he's requesting are "fighter jets, anti-aircraft and missile defence systems to protect our skies."

-ABC News' Will Gretsky

At least 4.8 million Ukrainian refugees are now scattered across 44 countries in Europe, according to the United Nations High Commissioner for Refugees.

The UNHCR has recorded 7.3 million border crossings from Ukraine and another 2.3 million crossings back into the country.

"Although the security situation in Ukraine remains fragile, crossings back and forth have been recorded," the agency said in a statement. "Some cross to Ukraine to assess the situation, check property, visit family members or help them to leave. Others are going to Western Ukraine and areas around Kyiv and Chernihiv intending to stay. Many who have moved back have found their homes severely damaged and struggled to find jobs -- as the war continues to have a devastating economic impact -- and had no choice but to leave again."

-ABC News' Will Gretsky

The fight for the eastern Ukrainian city of Severodonetsk will determine the fate of the wider Donbas region, according to Ukrainian President Volodymyr Zelenskyy.

"Severodonetsk remains the epicenter of the confrontation in Donbas. We defend our positions, inflict significant losses on the enemy," Zelenskyy said late Wednesday in his nightly address. "This is a very fierce battle, very difficult. Probably one of the most difficult throughout this war. I am grateful to everyone who defends this direction. In many ways, the fate of our Donbas is being decided there."

After launching an invasion of neighboring Ukraine on Feb. 24, Russian forces failed to take control of the capital, Kyiv, and other major government centers as they faced tough resistance from Ukrainian troops. Russian forces then switched attention to Donbas, which comprises the self-proclaimed republics controlled by Moscow-backed separatists in eastern Ukraine's Luhansk and Donetsk oblasts.

Severodonetsk, an industrial hub, is the largest city still held by Ukrainian troops in contested Donbas. In recent days, Russian forces have encircled the city as they advanced in the region, creating a pocket that could trap Ukrainian defenders there and in the neighboring city of Lysychansk.

Severodonetsk and Lysychansk are the last major cities in the Luhansk area still controlled by Ukraine. Last week, the U.K. Ministry of Defense said in an intelligence update that Russian forces had seized most of Severodonetsk, but that the main road into the pocket likely remained under Ukrainian control.

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan