Kyiv/Kramatorsk - Former Russian President Dmitry Medvedev said attempts by the West to punish a nuclear power such as Russia for the war in Ukraine risked endangering humanity, as the near five-month conflict leaves cities in ruins and thousands homeless.

Russia's Feb. 24 invasion of Ukraine has triggered the most serious crisis in relations between Russia and the West since the 1962 Cuban Missile Crisis, when many people feared the world was on the brink of nuclear war.

U.S. President Joe Biden says Russian President Vladimir Putin is a war criminal and has led the West in arming Ukraine and imposing crippling sanctions on Russia.

"The idea of punishing a country that has one of the largest nuclear potentials is absurd. And potentially poses a threat to the existence of humanity," Medvedev, now deputy chairman of the Russian Security Council, said on Telegram on Wednesday.

Russia and the United States control about 90% of the world's nuclear warheads, with around 4,000 warheads each in their military stockpiles, according to the Federation of American Scientists.

On Sunday, Putin claimed his biggest victory when Ukrainian forces withdrew from Luhansk province. Russian forces then launched an offensive to take neighbouring Donetsk province. Donetsk and Luhansk comprise the Donbas.

In the Donetsk city of Kramatorsk, which Russian forces are expected to try to capture in coming weeks, Ukrainian soldiers and a handful of civilians ran errands in green-painted cars and vans on Wednesday. Much of the population has left.

Outside the Donbas, Ukraine's second largest city Kharkiv was being subjected to "constant" longer-range Russian shelling, Mayor Ihor Terekhov said on Ukrainian TV.

Residents of Slovyansk, a key city in Ukrainian hands, were earlier urged to evacuate further west - a day after a deadly Russian attack on a local market.

Ukrainian President Volodymyr Zelensky, meanwhile, said "artillery from our Western partners had started working very powerfully, so the losses of the occupiers will only increase".

Ukraine says it is investigating more than 21,000 war crimes and crimes of aggression allegedly committed by Russia since the start of its invasion.

The International Criminal Court has described Ukraine as a "crime scene", dispatching its largest team of detectives ever to the country to assist in multiple investigations.

Moscow has repeatedly denied targeting civilians.
-Reuters/BBC