Ukrainian President Volodymyr Zelenskyy called Friday's deadly missile attack on the Kramatorsk train station in eastern Ukraine "another war crime of Russia."

"All the world's leading powers have already condemned Russia's attack on Kramatorsk," Zelenskyy said during his latest national address Friday. "We expect a firm, global response to this war crime."

The president called for the missile strike, which killed at least 50, to be among the charges in a war crimes tribunal against Russia.

"All the efforts of the world will be aimed to establish every minute: who did what, who gave orders. Where did the rocket come from, who was carrying it, who gave the order and how the strike was coordinated," Zelenskyy said. "Responsibility is inevitable."

During his daily press briefing, Pentagon spokesman John Kirby was clear that the U.S. believes that Russia was behind the rocket attack on a train station in eastern Ukraine earlier Friday.

"We find unconvincing Russian claims that they weren't involved, particularly when the ministry actually announced it and then when they saw reports of civilian casualties they decided to unannounce it,” Kirby said during Friday afternoon's briefing. "So our assessment is that this was a Russian strike and they used a short-range ballistic missile to conduct it."

Kirby called it a part of the trend by Russia of "brutality" and "carelessness" in not avoiding civilian casualties as they carry out this war in Ukraine.

At least 50 people, including five children, were killed in the rocket attack in Kramatorsk, Ukrainian authorities said.

A Kremlin spokesman denied involvement in the attack, saying Russia's Armed Forces do not use the type of missile used in the strike and that no combat tasks were planned in the region.

-ABC News' Luis Martinez

European Union President Ursula Von der Leyen was seen on camera witnessing a mass grave in Bucha, Ukraine, during a visit to the demolished city with the EU's chief diplomat Josep Borrell and Ukrainian Prime Minister Denys Shmyhal.

After touring Bucha, Von der Leyen met with Ukrainian President Volodymyr Zelenskyy and handed him an envelope with a questionnaire, marking the first step toward membership in the EU. Zelenskyy said he'd have responses in one week.

Global officials have accused Russian troops of committing war crimes after graphic images emerged of civilians lying dead in the streets of Bucha following the withdrawal of Russian forces.

At a press conference Thursday Secretary of State Antony Blinken said, "The sickening images and accounts coming out of Bucha and other parts of Ukraine have only strengthened our collective resolve and unity."

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

President Joe Biden on Thursday announced a new $800 million security assistance package to help Ukrainians fighting in the eastern Donbas region. This funding includes heavy artillery weapons, tactical drones and howitzers.

This package includes 72 new howitzers. Combined with the 18 howitzers announced last week, the 90 howitzers headed to Ukraine can equip five battalions, according to the administration.

Biden noted that he’s run out of pre-approved money and equipment to send to Ukraine and he appealed to Congress to approve more.

When pressed by ABC News on how long the U.S. can sustain this level of spending, Biden said, “We have the capacity to do this for a long time."

He went on, "The question is, are we going to continue to maintain the support of the international community? And keep the pressure on Putin to prevent him from overrunning the country, No. 1, and No. 2, make sure we continue to maintain the economic sanctions, which, over time, and we're beginning to see, they’re devastating their economy and their ability to move forward."

Biden also announced a separate $500 million in economic aid to Ukraine to help fund government operations like salaries, pensions and social programs.

The president added that the U.S. is now banning Russian-affiliated ships from American ports.

Biden met with Ukrainian Prime Minister Denys Shmyhal Thursday morning ahead of his remarks.

-ABC News' Luis Martinez and Sarah Kolinovsky