The mother of a Ukrainian soldier mourns over his flag-draped coffin in Lyiv on Wednesday. AFP

Ukraine braced for a climactic battle for control of the besieged country's industrial east. The country wants sanctions that are economically destructive enough for Russia to end its war after accusing some countries of still prioritising money over punishment for civilian killings that the West condemns as war crimes.

The mayor of the southern port city of Mariupol said Wednesday that more than 5,000 civilians had been killed there. Meanwhile, in areas north of the capital, Ukrainian officials gathered evidence of Russian atrocities amid telltale signs that Moscow’s troops killed people indiscriminately before retreating over the past several days.

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Russian forces withdrew from the shattered outskirts of Kyiv to regroup and intensify their offensive across the Donbas region, where authorities urged people to evacuate before time runs out.

Ukrainian soldiers walk next to destroyed Russian tanks and armoured vehicles in Bucha, in Kyiv region. Reuters

The democratic world must reject Russian oil and completely block Russian banks from the international finance system, President Volodymyr Zelensky said in his daily video address early on Thursday.

After grisly images of dead civilians in the streets of Bucha sparked international condemnation, Zelensky said Kremlin forces were trying to cover up evidence of atrocities.

"We have information that the Russian military has changed its tactics and is trying to remove people who have been killed from streets and basements ... this is just an attempt to hide the evidence and nothing more," Zelensky said, but did not provide evidence.

A destroyed building is pictured in Chernigiv, amid Russian invasion of Ukraine. AFP

Moscow has denied targeting civilians and says images of bodies in Bucha were staged to justify more sanctions against Moscow and derail peace talks.

In his nightly address, Zelensky warned that the Russian military continued to build up its forces in preparation for a new offensive in the east, where the Kremlin has said its goal is to "liberate” the Donbas, Ukraine’s mostly Russian-speaking industrial heartland. But he said Ukraine, too, was preparing for battle.

"We will fight and we will not retreat,” he said. "We will seek all possible options to defend ourselves until Russia begins to seriously seek peace. This is our land. This is our future. And we won’t give them up.”

Ukrainian soldiers are seen on tanks in Bucha, Kyiv region, Ukraine, on Wednesday. Reuters

Ukrainian authorities urged people living in the Donbas to evacuate immediately.

"Later, people will come under fire,” Deputy Prime Minister Iryna Vereshchuk said, "and we won’t be able to do anything to help them.”

A US defence official speaking on condition of anonymity said Russia had completed the pullout of all of its estimated 24,000 or more troops from the Kyiv and Chernihiv areas in the north, sending them into Belarus or Russia to resupply and reorganise, probably to return to the fight in the east.

But a Western official, also speaking on condition of anonymity to discuss intelligence estimates, said it will take Russia’s battle-damaged forces as much as a month to regroup for a major push on eastern Ukraine.