Sweden's ruling Social Democratic party announced Sunday that its board has decided to support the historically neutral country's bid to join NATO.

The party said it will now work to advance Sweden's application for membership to NATO.

If the application is approved by NATO, the Social Democratic Party said it will express unilateral reservations against the deployment of nuclear weapons and permanent bases on Swedish territory.

"We Social Democrats believe that the best for Sweden's and the Swedish people's security is that we join NATO," Swedish Prime Minister Magdalena Andersson said at a news conference Sunday. "This is a decision that was taken after careful deliberations. A position in favor of NATO means that we are prepared to abandon a security policy that Sweden has had in different forms over 200 years."

Andersson added, "For us Social Democrats it is clear that the military non-alignment has served Sweden well, but our conclusion is that it won't serve us as well in the future."

Leaders of Finland, another historically neutral Nordic country, also announced on Sunday that it will also apply for NATO membership.

-ABC News' Christine Theodorou

Figures show the number of people returning to Ukraine in the past three days is higher than the number of people trying to flee, the State Border Guard Service of Ukraine said on Sunday.

Of the nearly 84,000 people traveling in and out of Ukraine on Saturday, more than half were Ukrainian nationals returning to the country, the Ukrainian authorities said.

More than 46,000 people returned to Ukraine on Saturday while 37,000 people left the country, the Border Guard Service said.

At least 22,000 of those who left the country traveled to Poland while the rest went to Slovakia, Hungary, Romania and Moldova, officials said.

Since the start of the Russian invasion of Ukraine, more than 6 million Ukrainians have fled the country. Since then, more than 1.6 million people have returned to Ukraine, officials said.

-ABC News' Christine Theodorou

Finland's leaders on Sunday said the Nordic country would apply for NATO membership.

"It is a historic day -- of course, we have, for years, been in close partnership with NATO," Prime Minister Sanna Marin said on Sunday.

Marin and President Sauli Niinistö made the official announcement at a press conference in Helsinki, the capital. The Finnish Parliament is now expected to vote on whether to apply.

Marin said she hoped neighboring Sweden would also decide to join the military bloc in the coming days. Decisions made by both countries "will influence and affect the whole of Nordic countries," she said.

Niinistö said Finland has been discussing NATO membership internally for "at least 30 years."

"We have to keep in mind that NATO membership does not change geography, so we will always have that big border -- land and sea -- with Russians behind it," Niinistö said on Sunday.

Russia has "lost momentum and fallen significantly behind schedule" in its offensive on eastern Ukraine's Donbas region, the U.K. Ministry of Defense said.

"Despite small-scale initial advances, Russia has failed to achieve substantial territorial gains over the past month whilst sustaining consistently high levels of attrition," the ministry said on Sunday. "Russia has now likely suffered losses of one third of the ground combat force it committed in February."

Russian forces in late March pulled out of the suburbs north of Kyiv and collected in eastern Ukraine. They began an offensive in April, but have since foundered, the U.K. said.

Low troop morale, reduced combat effectiveness and loss of equipment have all played a role in slowing the Russian advance, the ministry said.

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