Ukraine has told Russia to release prisoners of war if it wants its most high-profile ally in the country freed, as Moscow sends its strongest signal yet the war will grind on.

US President Joe Biden on Tuesday referred to Russia’s attack on Ukraine as genocide for the first time, saying “we’ll let the lawyers decide internationally whether or not it qualifies, but it sure seems that way to me”.

Russia has repeatedly denied targeting civilians and has said Ukrainian and Western allegations of war crimes were made up to discredit Russian forces.

Ukraine announced on Tuesday that it had apprehended Viktor Medvedchuk, the Ukrainian tycoon who leads the Opposition Platform – For Life party.

“You can be a pro-Russian politician and work for the aggressor state for years. You may have been hiding from justice lately. You can even wear a Ukrainian military uniform for camouflage,” Ukraine’s security service said in a statement.

“But will it help you escape punishment? Not at all! Shackles are waiting for you and same goes for traitors to Ukraine like you.”

Mr Medvedchuk was reported to have escaped house arrest in February, just after Russia’s invasion of its neighbour began.

The 67-year-old pro-Russian figure, who says President Vladimir Putin is godfather to his daughter, has denied wrongdoing. He has repeatedly described the accusations against him as “political repression”.

For years Mr Medvedchuk had been tolerated in Ukrainian political circles because he was seen as an important channel of communication with the Kremlin. But he was charged with treason in May 2021.

“I propose to the Russian Federation: Exchange this guy of yours for our guys and girls now held in Russian captivity,” Ukraine’s President Volodymyr Zelensky said in an early morning address on Wednesday.

Hours earlier Mr Putin used his first public comments on the conflict in more than a week to insist Russia will “rhythmically and calmly” continue its operation, saying he was confident his goals, including on security, would be achieved.

Mr Zelensky mocked Mr Putin in his address.

“How could a plan that provides for the death of tens of thousands of their own soldiers in a little more than a month of war come about?” he said.

Mr Putin said that on-and-off peace negotiations “have again returned to a dead-end situation for us”.

Mr Putin, who had been ubiquitous on Russian television in the early days of the war, had largely retreated from public view since Russia’s withdrawal from northern Ukraine two weeks ago.

With the battle set to drag on, the US is expected to announce a further $US750 million in military assistance for Ukraine, two officials told Reuters.

“We urgently need more heavy weapons to prevent further Russian atrocities,” Mr Zelensky wrote on Twitter as he hailed Mr Biden’s comments on genocide.

Many of the towns Russia has retreated from in northern Ukraine were littered with the bodies of civilians killed in what Kyiv said was a campaign of murder, torture and rape.

Moscow denies the allegations.

Russia says it aims to capture more territory on behalf of separatists in two eastern provinces, known as the Donbas.

Their targets include Mariupol port, which has been reduced to a wasteland under Russian siege.

Ukraine says tens of thousands of civilians have been trapped inside that city with no way to bring in food or water, and accuses Russia of blocking aid convoys.

On Tuesday, Ukrainian marines were holed up in the Azovstal industrial district. Reuters journalists accompanying Russian-backed separatists saw flames billowing from the area.

Pavlo Kyrylenko, governor of the eastern Donetsk region – which includes Mariupol – said he had seen incident reports on possible chemical weapons use in the city but could not confirm them.

Ukraine said its forces in the east had beaten off six Russian attacks, destroying two vehicles and three artillery systems as well as shooting down a helicopter and two drones.

The US and Britain have said they were trying to verify reports claiming that Russia used chemical weapons.

On Wednesday, Mr Zelensky said it was not possible to draw 100 per cent firm conclusions about whether they had been used in Mariupol, due to the inability to carry out a proper probe.

Chemical weapons production, use and stockpiling is banned under the 1997 Chemical Weapons Convention.

Russia’s defence ministry has not responded to a Reuters request for comment. Russian-backed separatist forces in the east denied using chemical weapons in Mariupol, the Interfax news agency reported.

As Russia redoubles efforts in the east, Luhansk regional Governor Serhiy Gaidai urged residents to evacuate.

“It’s far more scary to remain and burn in your sleep from a Russian shell,” he wrote on social media.

-with AAP