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

Nate Mook, CEO of World Central Kitchen, opened up to ABC News Live on Monday about what he saw in the immediate aftermath of Friday's attack on a Ukrainian train station that killed at least 57 people.

Mook said Friday was the third day he was spending near the Kramatorsk train station planning food distribution for Ukrainians trying to flee the region.

On Friday, Mook said, "We had just driven by the station, I looked down and saw 1,000 people or so on the platform. And we got about two minutes beyond the station when we heard the explosions happen."

"We headed over there … the scene was horrific. It was catastrophic," Mook said. "There was damage both on the platform and in front of the station where innocent civilians were waiting … there was remnants of a rocket on the ground."

One of the areas that was really hit the hardest was actually a waiting area for seniors," Mook said. "They had a little waiting area set up, they had chairs, they had a little tent area. And this is right where the rocket landed and why so many were killed."

Mook added, "I think there was a little bit of shock around this idea that the train station itself, with just innocent civilians, would be targeted, because there's no strategic value to it -- it is just murder."

A small group of Ukrainian troops who were in the U.S. for pre-planned military training when Russia invaded their country returned home Sunday, a senior U.S. defense official told reporters Monday. The Pentagon took advantage of their presence to train them on the Switchblade drones the U.S. is sending to aid Ukraine.

The official said the U.S. is looking into training more Ukrainians, possibly in the U.S.

"There's been no policy decision that I'm aware of that would prevent them from coming to the United States," the official said.

But the Pentagon is considering several options.

"As for additional training on systems like the Switchblade, we are reviewing and thinking about and considering a number of different options for how we could manage to get more Ukrainians trained on that system," the official said.

-ABC News' Matt Seyler

Gen. Alexander Dvornikov has been appointed to lead Russia's invasion effort, a senior U.S. defense official told reporters Monday. Dvornikov previously led Russians in the south of Ukraine, where they saw the most success.

"In the early days and weeks of the conflict, the Russians did achieve more success in the south than they achieved certainly in the north. I have no idea that was a factor in his selection," the official said.

But the official noted that Russian efforts in the south are now largely stalled, with no progress against Mariupol since last week and no success advancing on Mykolayiv.

The official said it is unclear how Dvornikov's selection could affect the fighting but said a pattern of brutality remains constant.

"What is clear is that the Russians continued to sink to new lows of depravity and brutality, as we saw with the missile strike on the train station last week," the official said.

The official said some of the Russian troops that withdrew from northern Ukraine are starting to move east toward the Russian cities of Belgorod and Valuyki.

"We believe that this line of vehicles that we talked about that are north of Izium came out of the Belgorod/Valuyki region, from there to the south," the official said.

-ABC News' Matt Seyler

Russian President Vladimir Putin and Austrian Chancellor Karl Nehammer started negotiations Monday in the Russian presidential residence, Russia's semi-official Interfax reported, citing Kremlin spokesperson Dmitry Peskov.

This marked the first visit to Russia by an EU leader since the invasion started.

After the meeting Nehammer told reporters this wasn't a "friendly visit" with Putin and that he addressed the "serious war crimes in Bucha," The Austrian Press Agency reported. Nehammer said he told the Russian leader that the sanctions will escalate if Ukrainians keep dying, according to The Austrian Press Agency.