"Difficult" negotiations are underway between Ukraine and Russia to coordinate the evacuation of seriously wounded fighters from the Azovstal steel plant in Mariupol in exchange for an unspecified number of Russian prisoners of war, Ukrainian Deputy Prime Minister Iryna Vereshchuk said.

"We are currently negotiating only 38 severely wounded fighters that can't walk," she said.

-ABC News' Irene Hnatiuk

Russia "will be forced to take retaliatory steps both of military-technical and of other nature in order to stop the threats to its national security" in response to Finland’s possible accession to NATO, according to state-run TASS citing a statement by the Russian Foreign Ministry.

The ministry added, "Finland joining NATO will seriously harm bilateral Russian-Finnish relations and the maintenance of stability and security in the North European region."

Russia will conduct a "special analysis" and take necessary measures for national security if Finland joins NATO, Kremlin spokesman Dmitry Peskov said Thursday.

"There is an order from the president, the supreme commander-in-chief, that a list of measures be composed to strengthen our western flanks in connection with the strengthening of the eastern flank of NATO," Peskov told reporters during a daily briefing. "NATO has been enlarging towards us. So, clearly, all of that will be elements of a special analysis and the elaboration of measures necessary for maintaining a balance and providing our security."

"Everything will depend on the manifestation of the further [NATO] enlargement process -- how far the military infrastructure will grow towards our borders," he added.

When asked whether Finland's accession to NATO -- the world's biggest military alliance -- would pose a threat to Russia, Peskov told reporters: "Of course it will."

"Another enlargement of NATO does not make our continent more stable and secure," he said.

Peskov's comments followed a joint statement released by Finnish President Sauli Niinisto and Finnish Prime Minister Sanna Marin announcing their support for Finland applying to join NATO.

Finland's leaders announced Thursday their support for the country applying to join NATO, the world's biggest military alliance.

"During this spring, an important discussion on Finland's possible NATO membership has taken place. Time has been needed to let Parliament and the whole society establish their stands on the matter," Finnish President Sauli Niinisto and Finnish Prime Minister Sanna Marin said in a joint statement. "Time has been needed for close international contacts with NATO and its member countries, as well as with Sweden. We have wanted to give the discussion the space it required."

"Now that the moment of decision-making is near, we state our equal views, also for information to the parliamentary groups and parties. NATO membership would strengthen Finland's security," they added. "As a member of NATO, Finland would strengthen the entire defence alliance. Finland must apply for NATO membership without delay. We hope that the national steps still needed to make this decision will be taken rapidly within the next few days."

The Finnish Parliament will now take up the issue, with a vote expected to be held early next week.

The announcement marks a major shift in Finland's traditionally neutral policy, triggered by Russia's ongoing invasion of Ukraine. Finland shares an 830-mile border with Russia, and recent opinion polls show Finnish public support for joining NATO has surged to 76%.

Neighboring Sweden is also considering applying to join the alliance.

-ABC News' Ines de la Cuetara

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