The International Committee of the Red Cross said Thursday that it has registered hundreds of Ukrainian prisoners of war from a besieged steel plant in war-ravaged Mariupol this week, after the Ukrainian city fell into Russian hands.

A team from the ICRC began on Tuesday to register combatants leaving the Azovstal Iron and Steel Works plant, including the wounded, at the request of the parties to the conflict. The operation continued Wednesday and was still ongoing Thursday. The ICRC is not transporting prisoners of war to the places where they are held, according to a press release from the organization.

"The registration process that the ICRC facilitated involves the individual filling out a form with personal details like name, date of birth and closest relative," the organization said. "This information allows the ICRC to track those who have been captured and help them keep in touch with their families."

The Geneva-based humanitarian agency, which has been working in Ukraine since 2014, noted that it "maintains a confidential dialogue with the parties to the conflict on their obligations under international humanitarian law."

"In accordance with the mandate given to the ICRC by States under the 1949 Geneva Conventions, the ICRC must have immediate access to all POWs in all places where they are held," the organization added. "The ICRC must be allowed to interview prisoners of war without witnesses, and the duration and frequency of these visits should not be unduly restricted. Whenever circumstances permit, each party to the conflict must take all possible measures to search for and collect the dead."

For weeks, Ukrainian fighters and civilians were holed up inside Mariupol's vast Azovstal plant as the remaining pocket of Ukrainian resistance to Russia's relentless bombardment of the strategic southeastern port city. Russia claimed Thursday that 1,730 Ukrainian fighters had surrendered in Mariupol over the previous three days, while Ukraine confirmed Tuesday that more than 250 had yielded in the initial hours after it ordered them to do so.

Mariupol is the largest city that Russian forces have seized since launching an invasion of neighboring Ukraine on Feb. 24. Its complete capture gives Russia total control of the coast of the Sea of Azov as well as a continuous stretch of territory along eastern and southern Ukraine.

-ABC News' Max Uzol

Russia has fired senior military commanders in recent weeks "who are considered to have performed poorly during the opening stages of its invasion of Ukraine," the U.K. Ministry of Defense said Thursday in an intelligence update.

According to the ministry, Lt. Gen. Serhiy Kisel, who commanded Russia's elite 1st Guards Tank Army, has been suspended for his failure to capture Ukraine's second-largest city, Kharkiv. Vice Adm. Igor Osipov, who commanded Russia's Black Sea Fleet, has also likely been suspended following the sinking of the fleet's flagship, Moskva, in April. Gen. Valeriy Gerasimov, the Russian military's chief of the general staff, likely remains in his post, but it was unclear whether he retains the confidence of Russian President Vladimir Putin, according to the ministry.

"A culture of cover-ups and scape-goating is probably prevalent within the Russian military and security system," the ministry said. "Many officials involved in the invasion of Ukraine will likely be increasingly distracted by efforts to avoid personal culpability for Russia’s operational set-backs."

"This will likely place further strain on Russia's centralised model of command and control, as officers increasingly seek to defer key decisions to their superiors," the ministry added. "It will be difficult for Russia to regain the initiative under these conditions."

Russia has placed two Ukrainian commanders on a wanted list.

Serhiy Velychko and Kostiantyn Nemychev, commanders of the Azov Regiment, a far-right group now part of the Ukrainian military, have been added to the Russian Ministry of Internal Affairs' database of wanted fugitives in connection with a criminal case.

According to the Russian Investigative Committee, Velychko and Nemychev are accused of attempted murder of at least eight Russian servicemen who sustained multiple injuries in eastern Ukraine's Kharkiv region. Criminal charges were brought against the pair in absentia, and Russian authorities are working to track down and apprehend them.

The Senate on Wednesday night unanimously confirmed the new U.S. ambassador to Ukraine, Bridget Brink, a career foreign service officer.

The vote took place on the same day the U.S. officially resumed operations at its embassy in Kyiv.

-ABC News' Trish Turner

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