A grave with dozens of Ukrainians civilians has been found in Buzova village near Kyiv, an official said, the latest reported mass grave to be discovered as Russian forces retreat from their offensive on the capital and focus their assault on the east.

Taras Didych, head of the Dmytrivka community that includes Buzova, told Ukrainian television that the bodies were found in a ditch near a petrol station. The number of dead had yet to be confirmed.

"Now we are returning to life but during the occupation we had our 'hotspots', many civilians died," Didych said on Saturday.

Reuters was not immediately able to confirm the report.

Mounting civilian casualties have triggered a new wave of international condemnation, in particular over hundreds of deaths in the town of Bucha, to the northwest of Kyiv that until last week was occupied by Russian forces.

Ukraine and the West have accused Russian forces of war crimes in Bucha.

Russia has denied targeting civilians in what it calls a "special operation" to demilitarize and "denazify" its southern neighbour. Ukraine and Western nations have dismissed this as a baseless pretext for war.

Russia has failed to take one major city since the start of the invasion on Feb. 24 but Ukraine says Russia is gathering its forces in the east for a major assault and has urged people to flee.

Britain's defence ministry said Russia was seeking to establish a land corridor from Crimea, which it annexed in 2014, and the eastern Donbas region, which is partly held by Moscow-backed separatists.

Some cities there are under heavy shelling with tens of thousands of people unable to evacuate.

"This will be a hard battle, we believe in this fight and our victory. We are ready to simultaneously fight and look for diplomatic ways to put an end to this war," President Volodymyr Zelenskiy said in an address late on Saturday.

Zelenskiy said Russia's use of force was "a catastrophe that will inevitably hit everyone".

"Russian aggression was not intended to be limited to Ukraine alone ... the whole European project is a target for Russia," he said.

"Russia can still afford to live in illusions and bring new military forces and new equipment to our land. And that means we need even more sanctions and even more weapons for our state."

Zelenskiy called on the West to impose a complete embargo on Russian energy products and to supply Ukraine with more weapons.

British Prime Minister Boris Johnson met Zelenskiy in Kyiv on Saturday and pledged armoured vehicles and anti-ship missile systems, along with additional support for World Bank loans.

Britain will also ratchet up its sanctions on Russia and move away from using Russian hydrocarbons.

Johnson, speaking to reporters with Zelenskiy, said support for Ukraine was intended to ensure it "can never be bullied again, never will be blackmailed again, never will be threatened in the same way again".

Johnson was the latest foreign leader to visit Kyiv after Russian forces pulled back from the area.

The visits are a sign that Kyiv is returning to some degree of normality.

Some residents are coming back and cafes and restaurants are reopening. Italy said it planned to re-open its embassy this month.

NINE TRAINS

But in the east, calls by Ukrainian officials for civilians to flee have been given a greater sense of urgency by a missile attack on Friday on a train station in the city of Kramatorsk in the Donetsk region, crowded with women, children and the elderly trying to get out.

Ukrainian officials said more than 50 people were killed.

Russia has denied responsibility, saying the missiles used in the attack were only used by Ukraine's military. The United States says it believes Russian forces were responsible.

Reuters was unable to verify the details of attack.

Kramatorsk Mayor Oleksander Honcharenko said he expected just 50,000 to 60,000 of the city's population of 220,000 to remain as people flee.

Residents of the besieged region of Luhansk would have nine trains on Sunday to get out on, the region's governor, Serhiy Gaidai, wrote on the Telegram message service.

British military intelligence said that Russia's retreat from the capital region revealed "disproportionate" targeting of civilians.

Russia's invasion has forced about a quarter of Ukraine's 44 million people from their homes, turned cities into rubble and killed or injured thousands.

The European Union on Friday adopted new sanctions against Russia, including bans on the import of coal, wood, chemicals and other products. Oil and gas imports from Russia remain untouched.

Ukraine has banned all imports from Russia, a key trading partners before the war with annual imports valued at about $6 billion.

"The enemy's budget will not receive these funds, which will reduce its potential to finance the war," Economy Minister Yulia Svyrydenko wrote on her Facebook page.