A video released Sunday by a pro-Russian separatist commander showed a shower of bright-burning munitions cascading down on the Azovstal steel plant in the Ukrainian port city of Mariupol, where a few hundred Ukrainian fighters remain holed up weeks after the city fell into the hands of Russian forces.

The video shows projectiles bursting into showers of burning matter which then explode on contact with the ground or buildings.

ABC News has not been able to verify the authenticity of the video. Reuters was able to verify the location of the aerial footage to the Azovstal steel plant, but was not able to confirm the date and time the video was taken.

The footage was posted on Telegram by Alexander Khodakovsky, a commander of the pro-Russian self-proclaimed Republic of Donetsk.

"If you didn't know what it is and for what purpose -- you could say that it's even beautiful," Khodakovsky said in a message that accompanied the video.

It was not immediately clear what type of munitions were seen in the video.

Khodakovsky could not be reached for comment.

Ukrainian military officials said there was no letup on Sunday in Russia's bombardment of the steel works plant.

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.

White House national security advisor Jake Sullivan hinted Thursday of more sanctions coming against Russia in the "next week or two" aimed at targeting ways Moscow is evading sanctions already imposed.

“Where our focus will be over the course of the coming days is on evasion,” Sullivan said Thursday at the Economic Club of Washington. “As Russia tries to adjust to the fact that it’s under this massive economic pressure, what steps do they take to try to evade our sanctions and how do we crack down on that? And I think we'll have some announcements in the next week or two that identify targets that are trying to facilitate that evasion both inside Russia and beyond."

When Sullivan was asked whether sanctions will automatically be lifted if a negotiated peace deal between Russia and Ukraine is worked out, he appeared cautious with his words, saying, “a lot of that depends on what the shape and scope” of the agreement is.

“A lot of it depends on what the Ukrainians, in consultation with us and the Europeans come to agree to," Sullivan said. "You know, we're not going to do a deal over the head of the Ukrainians where we give a bunch of sanctions relief to Russia. But if some measure of sanctions relief were built in to some credible diplomatic solution led by the Ukrainians, that's something that we would happily discuss."

But Sullivan said Russian oligarchs shouldn't expect to ever get back their yachts and other assets seized under sanctions that have been imposed, saying the ultimate goal is "not to give them back” once the war is over.

“The president is actively looking at how we can deal with the fact that as we seize these assets, our goal is not to give them back. Our goal is to put them to a better use than that," Sullivan said. "But I'll be careful in what I say today because there's an ongoing kind of policy process around how we end up dealing with that question. But, rest assured, that the goal is not just to sit on them for a while."

-ABC News' Justin Gomez