A missile hit a train station where thousands of Ukrainians had gathered Friday, killing at least 50 and wounding dozens more in an attack on a crowd of mostly women and children trying to flee a new, looming Russian offensive in the country's east, Ukrainian authorities said.

The attack that some denounced as yet another war crime in the 6-week-old conflict came as workers unearthed bodies from a mass grave in Bucha, a town near Ukraine's capital where dozens of killings have already been documented following a Russian pullout.

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, heeding calls to leave the area before fighting intensifies in the Donbas region, the office of Ukraine's prosecutor-general said.

Ukrainian President Volodymyr Zelenskyy and other leaders accused Russia's military of deliberately attacking the station. Russia, in turn, blamed Ukraine, saying its forces don't use the kind of missile that hit the station - a contention 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.

“There are many people in a serious condition, without arms or legs,” Mayor Oleksandr Goncharenko said. Even with 30 to 40 surgeons working to treat them, the local hospital was struggling to cope, he said.

Britain's Defense Minister Ben Wallace denounced the attack as a war crime, and U.N. Secretary-General Antonio Guterres called it “completely unacceptable.”

“There are almost no words for it,” European Union Commission President Ursula von der Leyen, who is on a visit to Ukraine, told reporters. “The cynical behavior (by Russia) has almost no benchmark anymore.”

Ukrainian authorities and Western officials have repeatedly accused Russian forces of atrocities in the war that began with Russia's invasion on Feb. 24 and has since forced more than 4 million of Ukrainians to flee the country and displaced millions more.

Some of the most startling evidence of atrocities has come from towns around Ukraine's capital that Russian President Vladimir Putin's troops pulled back from in recent days.

In Bucha, Mayor Anatoliy Fedoruk has said investigators found at least three sites of mass shootings of civilians and were still finding bodies in yards, parks and city squares - 90 per cent of whom were shot.

Russia has falsely claimed that the scenes in Bucha were staged.

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.

About 67 people were buried in the grave, according to a statement from Prosecutor-General Iryna Venediktova's office, which is investigating the deaths, and other mass casualties involving civilians, as possible war crimes.

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. Twenty-six bodies were found there Thursday.

“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 killings around Kyiv were revealed after Russian forces pulled back after failing to take the capital in the face of stiff Ukrainian resistance. Russian troops have now 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.

A senior US defence official said Friday that the Pentagon believes some of the retreating units were so badly damaged that they are “for all intents and purposes eradicated.” The official spoke on condition of anonymity to discuss internal military assessments.

But the official did not say how many units sustained such extensive damage. Overall, the official said the U.S. believes Russia has lost between 15 per cent and 20 per cent of its combat power since the war began. While some combat units are withdrawing to be resupplied in Russia, Moscow has added thousands of troops around Ukraine's second-largest city, Kharkiv, he said.

The train station hit in Friday's missile strike is located in Ukrainian government-controlled territory, but Russia insisted it wasn't behind the attack. The Defense Ministry accused Ukraine of carrying it out in a statement carried by state news agency RIA Novosti, as did the region's Moscow-backed separatists, who work closely with Russian regular troops.

Kremlin spokesman Dmitry Peskov said the country's forces “do not use” the type of missile that hit the station - but experts said in fact they have used it during the war.