Ukrainian President Volodymyr Zelensky says about 30 people have been killed and 100 injured in a rocket strike on a railway station in the east of the country.

Writing on social media platforms, Mr Zelensky said thousands of people were at the station in Kramatorsk at the time of the strike.

Kramatorsk is a city in part of the Donetsk region controlled by the Ukrainian government, and the station was being used to evacuate civilians.

The strike came after Ukrainian leaders predicted more gruesome discoveries would be made in reclaimed cities and towns as Russian soldiers retreat to focus on eastern Ukraine.

Mr Zelensky accompanied a social media post with photos that showed a train carriage with smashed windows, abandoned luggage and bodies lying in what looked like an outdoor waiting area.

“The inhuman Russians are not changing their methods. Without the strength or courage to stand up to us on the battlefield, they are cynically destroying the civilian population,” he said.

“This is an evil without limits. And if it is not punished, then it will never stop.”

Russian-backed separatists in Donetsk claimed Ukrainian forces were responsible for the strike.

After failing to take Ukraine’s capital, the Kremlin has shifted its focus to 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.

Ukrainian officials warned residents this week to leave as soon as possible for safer parts of the country and said they and Russia had agreed to establish multiple evacuation routes in the east.

In his nightly video address, Mr Zelensky said: “And what will happen when the world learns the whole truth about what the Russian troops did in Mariupol?” The besieged southern port has seen some of the greatest suffering since Russia invaded Ukraine.

“There on every street is what the world saw in Bucha and other towns in the Kyiv region after the departure of the Russian troops. The same cruelty. The same terrible crimes.”

Nato nations have agreed to increase their supply of arms after Ukraine’s foreign minister pleaded for weapons from the alliance and other sympathetic countries to help face down an expected offensive in the east.

Bucha mayor Anatoliy Fedoruk said investigators found at least three sites of mass shootings of civilians during the Russian occupation.

He said most victims died from gunshots, not from shelling, and some corpses with their hands tied had been “dumped like firewood” into mass graves, including one at a children’s camp.

Mr Fedoruk said 320 civilians were confirmed dead as of Wednesday, but he expected more as bodies are found in the city that was home to 50,000 people.

In his nightly address, Mr Zelensky said Bucha’s horrors may be only the beginning. In the northern city of Borodyanka, 20 miles from Bucha, he warned of even more casualties, saying “there it is much more horrible”.

The weekly magazine Der Spiegel reported Germany’s foreign intelligence agency intercepted radio messages among Russian soldiers discussing killings of civilians. Moscow has falsely claimed the scenes in Bucha were staged.

A Kremlin spokesman said on Thursday that Russia has suffered major troop casualties during its six-week military operation in Ukraine.

“Yes, we have significant losses of troops and it is a huge tragedy for us,” Kremlin spokesman Dmitry Peskov told Sky News.

He hinted the fighting might be over “in the foreseeable future”, telling Sky that Russian troops were “doing their best to bring an end to that operation”.

The United Nations estimates the war has displaced at least 6.5 million people within the country.

The UN refugee agency, UNHCR, said that more than 4.3 million, half of them children, have left Ukraine since Russia launched its invasion on February 24 and sparked Europe’s largest refugee crisis since the Second World War.

The International Organisation for Migration estimates more than 12 million people are stranded in areas of Ukraine under attack.