Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

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

When Pentagon press secretary John Kirby Kirby was asked at Friday's briefing whether he believes Russian President Vladimir Putin is a rational actor, he first responded by saying he couldn't speak to his psyche, adding, "It's hard to look at what he's doing in Ukraine, what his forces are doing in Ukraine, and think that any ethical, moral individual could justify that."

Kirby then appeared to get choked up and paused for several seconds. He said, "Sorry, it's difficult to look at some of the images and imagine that any well-thinking, serious, mature leader would do that. So I can't talk to his psychology, but I think we can all speak to his depravity."

Kirby later apologized, saying, "I didn't mean to get emotional. I apologize for that. I don't want to make this about me. But I've been around the military a long, long time and I've known friends who didn't make it back. It's just hard."

He went on, "It's difficult to look at that and it's hard to square his -- let's just call it what it is, his BS: This is about Nazism in Ukraine, and it's about protecting Russians in Ukraine, and it's about defending Russian national interests when none of them, none of them were threatened by Ukraine. It's hard to square that rhetoric by what he's actually doing inside Ukraine to innocent people. Shot in the back of the head, hands tied behind their backs. Women, pregnant women being killed. Hospitals being bombed. I mean, it's just unconscionable."

Kirby announced at Friday's briefing that the U.S. has started "training with Ukrainian armed forces on key systems at U.S. military installations in Germany."

"These efforts build on the initial artillery training that Ukraine's forces have already received elsewhere, and also includes training on radar systems and armored vehicles that have been recently announced as part of security assistance packages," Kirby said.

-ABC News' Matt Seyler

Pentagon press secretary John Kirby told CNN Friday that Russian President Vladimir Putin "absolutely shouldn't be" at the G-20 summit, set for November in Bali.

"He has isolated Russia by his own actions and he should continue to be isolated by the international community," Kirby said.

"I can't speak for President Biden or what the schedule might offer for the president, for United States attendance. But it's inappropriate, I think, for the entire international community to keep treating Russia as if things were normal, because it's not," Kirby said.

-ABC News' Matt Seyler