Actress Angelina Jolie visited Lviv, Ukraine, on Saturday, meeting with officials and posing for photos with children at a railway station.

Jolie is a special envoy for the U.N. High Commissioner for Refugees. More than 12 million people have fled their homes in Ukraine and more than 5 million have fled to neighboring countries, U.N. Secretary-General Antonio Guterres said Friday.

The Lviv Regional State Administration shared photos of Jolie being informed about the situation in the area as did several residents of the city, including those at a bakery, Lviv Croissants, where the actress stopped.

In just two months, Ukrainian law enforcement agencies have received over 7,000 reports of disappearances. About half of them were found, according to Mary Akopyan, Ukraine's deputy minister of internal affairs.

The number of people in Ukraine who have disappeared due to the war is unprecedented in modern world history, Hakobyan claimed in a meeting with a delegation of the International Commission on Missing Persons, an intergovernmental organization that addresses the issue of missing persons as a result of armed conflict, human rights violations and natural disasters.

The government received 2,000 unrecognizable bodies, 1,282 of which were later identified, according to the ministry.

An ICMP group of specialists will be arriving in Kyiv in a few weeks to provide help in identifying victims, according to the ministry.

-ABC News' Somayeh Malekian

Pentagon press secretary John Kirby said Friday that Russia's military has weakened since invading Ukraine.

"They have suffered thousands of casualties. They have lost airplanes. They have lost tanks. They have certainly lost battles," he said.

Russian forces are now trying to avoid mistakes they made around Kyiv earlier in the invasion, but stiff Ukrainian resistance and a more cautious approach seem to be slowing their advance, a senior U.S. defense official said.

The Russians, who were plagued by fuel and food shortages during earlier fighting in the north, are now wary of getting too far ahead of their supply lines, the official said.

Another factor slowing their progress is that their tactic of launching artillery and airstrikes to soften areas before moving ground troops forward is not working well.

"Their ground movements are fairly plodding because the artillery and airstrikes that they're launching against Ukrainian positions are not having the effect that they want them to have," the official said. "Ukrainians are still able to resist."

The Pentagon believes Russian forces are behind schedule by "at least several days" on their various lines of approach, the official said.

"We believe they meant to be much further along in terms of the total encirclement of Ukrainian troops in the east, and they have not been able to link north with south. In fact, they're nowhere close to linking north and south as the Ukrainians continue to fight back," the official said.

But Russia retains certain advantages in the eastern Donbas region, where its forces have high numbers and benefit from shorter lines of communication because they're fighting closer to their own border.

And while there is already fighting in Donbas, the Pentagon believes Russia is still setting conditions "for a sustained and larger and longer offensive" in the region, the defense official said.

"It could go on for some time. We've described it as a potential knife fight, and I think it's beginning to shape up to be exactly that," the official said.

Almost 20 shipment flights have arrived from seven different nations in the last 24 hours carrying mines, small arms ammunition, rockets and body armor, according to the official.

Over the next 24 hours, more than 12 flights carrying U.S. military aid for Ukraine are expected to arrive in the region, including howitzers, 155mm artillery rounds and the first shipment of Phoenix Ghost drones, the official said.

-ABC News' Matt Seyler

U.S. citizen Willy Joseph Cancel was killed in Ukraine while fighting alongside Ukrainian troops against invading Russian forces, his family confirmed to ABC News early Friday. The news was first reported by CNN.

Cancel, a 22-year-old former U.S. Marine, "was eager to volunteer" when he learned about the war in Ukraine, according to his wife, Brittany Cancel.

"He went there wanting to help people, he had always felt that that was his main mission in life," Brittany Cancel told ABC News in a statement. "My husband was very brave and a hero."

Before going to Ukraine, Cancel was working as a detention officer in Kentucky. He also had dreams of becoming a police officer or firefighter, according to his wife.

"I did not expect to be a widow at 23 years old or for our son to be without a father," she said. "All I want is for him to come home, and to give him the proper burial he deserves."

An official with the U.S. Department of State told ABC News on Friday morning that they "are aware of these reports and are closely monitoring the situation," but declined to comment further "due to privacy considerations."

State Department spokesperson Ned Price told MSNBC later on Friday that the department is "in the process of reaching out to the family ... to learn more details, to ascertain how we might be in a position to best support the family."

White House press secretary Jen Psaki expressed her condolences to Cancel's family at Friday's briefing, saying he "certainly sounded like a very passionate young man."

"A wife is mourning and our hearts are with them," she said.

Psaki also urged Americans not to travel to Ukraine.

"We know people want to help, but we do encourage Americans to find other ways to do so rather than traveling" to Ukraine, she said.

-ABC News' Caroline Guthrie and Conor Finnegan

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn