Attempts by the West to punish a nuclear power such as Russia for the war in Ukraine risks endangering humanity, the country’s former president Dmitry Medvedev says.

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

US President Joe Biden has called Russian leader Vladimir Putin 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,” Mr Medvedev, now deputy chairman of the Russian Security Council, said on social media messaging app Telegram on Wednesday.

“And potentially poses a threat to the existence of humanity”.

Russia and the United States control about 90 per cent of the world’s nuclear warheads, with around 4000 each in their military stockpiles, according to the Federation of American Scientists.

Mr Medvedev cast the United States as an empire which had spilled blood across the world, citing the killing of Native Americans, US nuclear attacks on Japan and a host of wars ranging from Vietnam to Afghanistan.

Attempts to use courts or tribunals to investigate Russia’s actions in Ukraine would, Mr Medvedev said, be futile and risk global devastation.

Ukraine and its Western allies say Russian forces have engaged in war crimes.

After failing to seize the capital Kyiv early in its invasion, Russia is now waging a war of attrition for Ukraine’s Donbas region, parts of which are controlled by separatist proxies loyal to Moscow.

On Sunday, Mr Putin claimed his biggest victory when Ukrainian forces withdrew from Luhansk province.

Russia then launched an offensive to take neighbouring Donetsk province, which together with Luhansk makes up the Donbas.

Russia says it wants to wrest control of the heavily industrial region on behalf of the separatists in two self-proclaimed people’s republics.

On Wednesday, the Ukrainian military said it had so far staved off any major Russian advance into the north of Donetsk, but pressure is intensifying with heavy shelling of the city of Sloviansk and nearby areas.

It said Russian forces were bombarding several Ukrainian towns with heavy weaponry to enable ground forces to close in on Sloviansk.

“The enemy is trying to improve its tactical position … (They) advanced … before being repulsed by our soldiers and retreating with losses,” the Ukrainian military said in its evening note.

Other Russian forces, it said, aimed to seize two towns en route to the city of Kramatorsk, south of Sloviansk, and were also trying to take control of the main highway linking Luhansk and Donetsk.

“We are holding back the enemy on the (Luhansk/Donetsk) border,” Luhansk Governor Serhiy Gaidai told Ukrainian TV.

Later, he said Luhansk was still not entirely occupied by Russian forces and Russia had sustained “colossal losses”.

Sloviansk Mayor Vadym Lyakh told a video briefing the city had been shelled for the last two weeks.

“The situation is tense,” he said.

Ukraine has repeatedly pleaded with the West to send more weapons to repel the invasion that has killed thousands, displaced millions, and flattened cities.

“At last, Western artillery has started to work powerfully, the weapons we are getting from our partners. And their accuracy is exactly what is needed,” Ukrainian President Volodymyr Zelensky said in his nightly video message.

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.

“It’s almost deserted. It’s spooky,” said Oleksandr, a 64-year-old retired metal worker. He was unlikely to follow official advice to evacuate, he said, despite an increase in missile strikes.

“I’m not looking for death but if I encounter it it’s better to be at home,” he said.

Outside the Donbas, Ukraine’s second largest city Kharkiv was being subjected to “constant” long-range Russian shelling, Mayor Ihor Terekhov said on Ukrainian TV.

“Russia is trying to demoralise Kharkiv. It won’t get anywhere,” he said.

– AAP