Serhiy Volyna, commander of Ukraine's 36th Marine Brigade, which is engaged in a brutal fight to defend Mariupol against invading Russian forces, has issued an urgent plea for military reinforcements or a political solution -- anything to break Russia's siege of the Ukrainian port city.

In an interview Friday with Ukrainian online newspaper Ukrayinska Pravda, the commander said the situation is critical and the fighting is fierce.

"It can be done and it must be done as soon as possible," Volyna added.

Despite Russia's relentless bombardment for more than a month, the 36th Marine Brigade along with units of the Azov Regiment, a far-right group now part of the Ukrainian military, have held down Mariupol. They have refused to surrender, vowing to fight until the end.

-ABC News' Yulia Drozd

During his latest daily address, Ukrainian President Volodymyr Zelenskyy observed that Ukraine has withstood 50 days of the Russian invasion.

"During the 50 days of this war, Ukraine became a hero for the whole free world," he said.

Zelenskyy said he was grateful to those who have supported the country, though said that during the last 50 days he's started to view world leaders in a different light.

"I have seen politicians behaving as if they had no power, and I have seen non-politicians who did more in these 50 days than some statesmen who claimed leadership," he said.

Zelenskyy praised how Ukraine has defended itself, including those "who have shown that Russian ships can go ... to the bottom only" -- a nod to the sinking of the Russian Black Sea Fleet flagship vessel, Moskva.

Ukrainian government officials had claimed on Wednesday its armed forces fired missiles that damaged the vessel.

-ABC News' Fidel Pavlenko

The U.S. Department of State's s ongoing review of atrocities in Ukraine could conclude with a determination that genocide has been committed by Russian forces, spokesperson Ned Price said Thursday.

The comments come after President Joe Biden described Russian President Vladimir Putin's actions in Ukraine as genocide.

Price told reporters during a briefing Thursday that there was some "misimpression" about the process, but that the department's effort to document, compile and analyze evidence of atrocities, which determined last month that Russian forces were committing war crimes, could include a determination on genocide.

"That same broader process the process to collect, analyze, share, document evidence of atrocities and potential atrocity crimes is the very same one that could ultimately inform other potential atrocity crime determinations, including the atrocity crime of genocide," Price said.

Price reiterated that the U.S. is working with the Ukrainian prosecutor-general, whose office has clear jurisdiction for potential war crimes trials.

The U.S. has been providing the Ukrainian prosecutor-general's office with the evidence it has collected, though it has not yet provided that information to the International Criminal Court, of which it's not a member.

Meanwhile, the International Criminal Court's chief, Karim A.A. Khan, currently is in Ukraine surveying scenes of atrocities in the Kyiv suburbs of Bucha and Borodyanka.

-ABC News' Shannon Crawford and Conor Finnegan

France is planning to move its embassy back to Kyiv "very soon" after relocating it more than 500 miles away in Lviv in western Ukraine when hostilities began to heat up around the capital city in March, the French foreign minister said.

French Foreign Minister Jean-Yves Le Drian made the announcement in a phone call Thursday with his Ukrainian counterpart, Dmytro Kuleba.

"This redeployment will take place very soon and will make it possible to further deepen the support provided by France to Ukraine in all areas to deal with the war launched by Russia on February 24," Le Drian said in a statement.

U.S. State Department spokesperson Ned Price was asked at a briefing if the United States was considering a similar move now that the bulk of the fighting has shifted in Ukraine from the south of the country to the east. The U.S. embassy in Ukraine was also moved from Kyiv to Lviv following the start of the Russian invasion, but embassy staff has been working from Poland in recent days, Price said.

Price told reporters that the State Department is "always reviewing" the possibility of moving the embassy back to Kyiv, but the embassy team remains in Poland and is not crossing the border into Lviv as they had been.

"Obviously, our goal is to have a functioning diplomatic presence in Ukraine as soon as it is safe and practical for us to do so," Price said.

-ABC News' Conor Finnegan

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