KYIV, Ukraine — A missile hit a train station where thousands of people had gathered to flee in eastern Ukraine, killing at least 50 on Friday, Ukrainian authorities said, as workers unearthed bodies from a mass grave in a town that has become the center of war crimes allegations against Russian troops.

Photos from the station in Kramatorsk showed the dead covered with tarps on the ground and the remnants of a rocket with the words “For the children” painted on it in Russian. About 4,000 civilians were in and around the station at the time of the strike, the office of Ukraine’s prosecutor-general said, adding that most were women and children heeding calls to leave the area before Russia launches a full-scale offensive in the country’s east.

Ukrainian President Volodymyr Zelenskyy and other leaders accused Russia’s military of deliberately attacking the station in a city in Ukraine’s contested Donbas region. Russia, in turn, blamed Ukraine, saying its forces don't use the kind of missile that hit the station — a contention military experts dismissed.

“Without the strength or courage to stand up to us on the battlefield, (Russian troops) are cynically destroying the civilian population,” Zelenskyy said on social media. “This is an evil without limits. And if it is not punished, then it will never stop.”

Pavlo Kyrylenko, the regional governor of Donetsk, which lies in the Donbas, said that 50 people were killed, including five children, and many dozens more were wounded.

Even with 30 to 40 surgeons working to treat the wounded, the local hospital was struggling to cope, Mayor Oleksandr Goncharenko said.

“There are many people in a serious condition, without arms or legs,” he said.

Britain’s Defense Minister Ben Wallace denounced the attack as a war crime and European Union Commission President Ursula von der Leyen called it “atrocious.”

“There are almost no words for it,” said von der Leyen, who is on a visit to Ukraine. “The cynical behavior (by Russia) has almost no benchmark anymore.”

Ukrainian authorities and Western officials have repeatedly accused Russian forces of war crimes in the six-week war that has also forced more than 4 million of Ukrainians to flee the country. Some of the most horrific evidence of atrocities has come from towns around Ukraine's capital that President Vladimir Putin's troops pulled back from in recent days.

In one of those towns, Bucha, journalists and returning Ukrainians have found scores of bodies lying in the streets, some with their hands bound and others burned.

On Friday, workers pulled corpses from a mass grave near a church in the town under spitting rain, lining up black body bags in rows in the mud. The office of Prosecutor-General Iryna Venediktova, who was visiting the town, said about 67 people were buried in the grave.

Many have bullet wounds, she said.

“What does this mean? This means that they killed civilians, shot them,” said Venediktova, whose office is investigating the deaths, and other mass casualties involving civilians, as possible war crimes.

The town's mayor, Anatoliy Fedoruk, said investigators found at least three sites of mass shootings of civilians and were still finding bodies in yards, parks and city squares.

“Ninety percent of the civilians died from gunshots and not from shelling,” he said Thursday on Ukrainian television.

In his nightly video address, Zelenskyy warned that more horrors could yet be revealed. Already, he said atrocities worse than the ones in Bucha had surfaced in Borodyanka, another settlement outside the capital.

“And what will happen when the world learns the whole truth about what the Russian troops did in Mariupol?” Zelenskyy said late Thursday, referring to the besieged southern port that has seen some of the greatest suffering during Russia’s invasion.

The prosecutor-general also expressed concern about the death toll in Borodyanka, where the process of retrieving bodies from shelled and collapsed buildings has just begun. Twenty-six bodies were found Thursday from the ruins of just two buildings, Venediktova said.

The killings were revealed after Russian forces pulled back from the capital after failing to take the city in the face of stiff Ukrainian resistance. Russian troops are now regrouping and have set their sights on the Donbas, a mostly Russian-speaking, industrial region in eastern Ukraine where Moscow-backed rebels have been fighting Ukrainian forces for eight years and control some areas.

The train station hit in Friday's missile strike is located in government-controlled territory, but Russia insisted they weren't behind the attack. Moscow-backed separatists, who also operate in the region and work closely with Russian regular troops, also blamed Ukraine for the strike.

Kremlin spokesman Dmitry Peskov said the country’s forces “do not use” the type of missile that hit the station.

Military experts dismissed that, saying Russia has already used the same type of missile during the war and has the only logical motive for attacking a rail station at this stage of the war.

One analyst said only Russia would have a reason to attack civilian railway infrastructure in the Donbas, and that Ukraine would not deliberately kill its own civilians in “a war of survival.”

“The Ukrainian military is desperately trying to reinforce units in the area … and the railway stations in that area in Ukrainian-held territory are critical for movement of equipment and people,” said Justin Bronk, a research fellow at the Royal United Services Institute in London.

Bronk said Russia’s use of the missile that hit the station has been documented during the current war and he pointed to other occasions that Russian authorities have tried to deflect blame by claiming their forces no longer use an older weapon “to kind of muddy the waters and try and create doubt.”

He suggested that Russia specifically chose the missile type because the Ukrainian army also has it, as “a pre-planned measure to allow them to trot out this same old line of ‘We don’t use that system, it's an old system and just muddy the waters continually.'"

The Donbas, which includes both the Donetsk and Luhansk regions, is bracing for a coming onslaught.

The governor of Luhansk, Serhiy Haidai, said Russia was concentrating equipment and troops and increasing shelling and bombing to aid their advance.

“We sense the end of preparations for that massive breakthrough, for that great battle which will happen here around us,” he said in a televised address.

Ukrainian officials have pleaded with Western powers to send more arms — and further punish Russia with sanctions — in order to stop the offensive. NATO nations agreed Thursday to increase their supply of arms, and Slovakian Prime Minister Eduard Heger announced on a trip to Ukraine on Friday that his country has donated its Soviet-era S-300 air defense system to Ukraine.

Later, Slovak Defense Minister Jaroslav Nad said the U.S. would deploy a Patriot air defense system to Slovakia for as long as needed, a precondition to give the S-300 long-range air missile system to Ukraine.

Zelenskyy had mentioned the S-300s by name when he spoke to U.S. lawmakers by video in March, appealing for anti-air systems that would allow Ukraine to “close the skies” to Russian warplanes and missiles.

Heger accompanied von der Leyen, the EU Commission president, and EU foreign policy chief Josep Borrell on a trip to Kyiv, part of efforts to signal the EU’s support for Ukraine.

In anticipation of intensified attacks by Russian forces, hundreds of Ukrainians fled villages in the Mykolaiv and Kherson regions that were either under attack or occupied.

“They are waiting for a big battle. We saw shells that did not explode. It was horrifying,” said Marina Morozova, who fled Kherson with her husband. The city was the first major one to fall to the Russians and is the scene of continued fighting as Ukrainians work to re-take it.

Morozova, 69, said only Russian television and radio was available. The Russians handed out humanitarian aid, she said, and filmed the distribution.

Russia expelled 45 Polish diplomats on Friday, retaliation for Poland’s March decision to expel the same number of Russians it said were spies with diplomatic cover — part of a series of expulsions of Russian diplomats from Western countries.