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

The Pentagon is "not buying" Russia's denial of responsibility for the Ukraine train station attack that killed at least 50, a senior U.S. defense official said Friday.

"They originally claimed a successful strike and then only retracted it when there were reports of civilian casualties," the official said. "It's our full expectation that this was a Russian strike -- we believe they used a short-range ballistic missile, an SS-21."

As Russian troops retreat from some Ukrainian cities, some of the Russian battalion tactical groups (or BTGs, with roughly 800-1,000 troops each) that have withdrawn back across the Belarusian and Russian borders have been essentially "eradicated" from the fighting in Ukraine, according to the senior defense official.

"There's just nothing left of the BTG except a handful of troops and maybe a small number of vehicles," the official said.

In terms of total losses -- counting troops, tanks, aircraft and missile inventory -- Russia has lost between 15-20% of the combat power it originally had arrayed against Ukraine since the beginning of the invasion, according to the official.

According to the official, some of the withdrawn Russian forces are now making their way to the Russian cities of Belgorod and Valuyki, to the northwest of Donbas. But there are "no indications" that fresh troops are waiting there to join them, the official said.

For now, degraded Russian BTGs are "exploring the option of" consolidating, banding together remaining troops and supplies to form new units, the official said.

Russia is also aiming to recruit upwards of 60,000 new troops, according to the official.

After Russian BTGs rebuild, "the most likely course of action would be for them to move immediately south right into the Donbas," the official said.

The Pentagon now estimates more than 40 Russian BTGs are positioned in or near the Donbas region. The estimate was "more than 30" on Wednesday, meaning up to 10,000 more troops have arrived in recent days.

-ABC News' Matt Seyler

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan

At least 50 people, including five children, were killed in a rocket attack on a railway station in eastern Ukraine on Friday, authorities said.

Two Russian rockets struck the train station in the city of Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company, which in a statement via Facebook called the attack "a purposeful strike on the passenger infrastructure of the railway and the residents of the city of Kramatorsk."

Donetsk Oblast Gov. Pavlo Kyrylenko said 38 of the 50 killed died at the scene while 12 died in hospitals. At least 100 were injured, according to the governor.

The attack occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko, who accused Russian forces of "deliberately trying to disrupt the evacuation of civilians."

"The evacuation will continue," the governor added. "Anyone who wants to leave the region will be able to do so."

Graphic images provided by Ukrainian officials showed the aftermath of the attack -- bodies lying on the ground next to scattered luggage and debris, with charred vehicles parked nearby. The remains of a large rocket with the words "for our children" in Russian painted on the side was also seen on the ground next to the main building of the station. Ukrainian President Volodymyr Zelenskyy said in a statement via Facebook that a Tochka-U short-range ballistic missile was used in Friday's attack.

Earlier this week, large crowds of people were seen waiting on the platform to board trains at the Kramatorsk railway station as they fled the city in Ukraine's disputed Donbas region.

Since 2014, Russia-backed separatist forces have controlled two breakaway republics of the Donetsk and Luhansk oblasts in the Donbas. The separatists have been fighting alongside Russian troops to seize more territory there, after Russia launched its invasion of Ukraine on Feb. 24. Now, the Russian military is said to be refocusing its offensive in the Donbas as its troops withdraw from northern Ukraine.

A senior U.S. administration official told ABC News that Russia isn't informing the majority of families when someone is killed in the war.

The official said mothers and spouses are starting to show up outside military bases to try to get information but are told to leave.

The official said mobile crematoriums are being used to burn the bodies of some Russian soldiers.

-ABC News' Martha Raddatz