Ukrainian officials are focusing on clearing the mines left behind by Russian forces before they retreated the region surrounding Kyiv, Ukrainian President Volodymyr Zelenskyy announced in his address Monday.

The Russian troops deliberately left mines "everywhere," Zelenskyy said, adding that they did so "to kill or maim as many of our people as possible, even when they were forced to withdraw from our land."

"Russian troops left behind tens, if not hundreds of thousands, of dangerous objects," Zelenskyy said.

Zelenskyy said Ukrainian officials are also focusing on procuring more arms. The country is not getting the lethal aid it needs to end the war sooner, he said.

The president also called on the European Union to include an oil embargo in its sixth round of sanctions toward Russia, saying that unless they do, another round of sanctions will be received by Moscow "with a smile."

-ABC News' Christine Theodorou

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

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