Finland and Sweden formally submitted their applications to join NATO to Secretary General Jens Stoltenberg Wednesday morning at the alliance’s Brussels headquarters.

Stoltenberg welcomed the requests, saying, "This is a good day, at a critical moment for our security," according to NATO.

"Every nation has the right to choose its own path. You have both made your choice, after thorough democratic processes. And I warmly welcome the requests by Finland and Sweden to join NATO," Stoltenberg said Wednesday.

Adding, "You are our closest partners. And your membership in NATO would increase our shared security."

Russian Sgt. Vadim Shyshimarin pleaded guilty Wednesday to shooting a 62-year-old Ukrainian man on Feb. 28. The guilty plea carries a life sentence.

It's the first trial Ukraine has conducted for an act that could be considered a war crime.

Asked by the presiding judge whether he accepted his guilt, Shyshimarin said: "Yes. Fully yes."

Kremlin spokesman Dmitri Peskov dismissed the proceedings on Wednesday, telling reporters that accusations leveled against Russian soldiers by Ukraine were "simply fake or staged."

-ABC News' Joe Simonetti

As Turkey becomes more vocal about its opposition to Sweden and Finland joining NATO, the State Department said it is still assured of the alliance's unified support for the two prospective members.

State Department spokesperson Ned Price said during a briefing Tuesday that Secretary of State Antony Blinken came away from meetings with NATO allies with a "sense of confidence there was strong consensus for admitting Finland and Sweden into the alliance if they so choose to join, and we're confident we'll be able to preserve that consensus."

Price said that assessment came from what Blinken heard in conversations behind closed doors.

Turkey's president, Recep Tayyip Erdogan, has publicly said that both candidates are untrustworthy because he perceives them as being supportive of groups Ankara views as extremist.

There is speculation that Turkey’s opposition is an attempt to leverage the moment to achieve its own policy goals or concessions from the U.S. Price said Tuesday that Turkey has not made any specific requests.

Price confirmed that Blinken will meet with his Turkish counterpart on the sidelines of the U.N. on Wednesday, adding that "other conversations are ongoing between and among current NATO allies and potential aspirant countries."

-ABC News' Shannon Crawford

Swedish Prime Minister Magdalena Andersson's office announced Sweden and Finland will jointly submit an application for NATO membership on Wednesday, after she met with Finnish President Sauli Niinistö in Stockholm.

"It is a message of strength and a clear signal that we stand united going into the future," Andersson said in a joint press conference with the Finnish president.

The two leaders are set to meet President Joe Biden at the White House on Thursday.

The two countries have stepped away from nonalignment in the wake of Russian's invasion of Ukraine, and fears for their own security.

-ABC News' Christine Theodorou

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