LVIV. US President Joe Biden said for the first time that Moscow's invasion of Ukraine amounts to genocide, as President Vladimir Putin said Russia would "rhythmically and calmly" continue its operation and achieve its goals.

"Yes, I called it genocide because it has become clearer and clearer that Putin is just trying to wipe out the idea of being able to be Ukrainian and the evidence is mounting," Biden told reporters as he prepared to board Air Force One on Tuesday.

Biden has repeatedly called Putin a war criminal, but delivering a speech at an ethanol plant in Iowa earlier on Tuesday the US president escalated his rhetoric to accuse Russia of genocide.

"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 are concocted.

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

Interfax Ukraine news agency on Wednesday quoted the Kyiv district police chief saying 720 bodies have been found in the region around the capital, with more than 200 people missing.

The Kremlin says it launched a "special military operation" on Feb. 24 to demilitarise and "denazify" Ukraine. Kyiv and its Western allies reject that as a false pretext for the invasion.

Moscow's nearly seven-week long incursion, the biggest attack on a European state since 1945, has seen more than 4.6 million people flee abroad, killed or injured thousands and left Russia increasingly isolated on the world stage.

Putin on Tuesday used his first public comments on the conflict in more than a week to say Russia would "rhythmically and calmly" continue its operation, and expressed confidence his goals, including on security, would be achieved.

Ukrainian President Volodymyr Zelenskiy mocked Putin in an early morning address on Wednesday: "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?"

Putin said that on-off peace negotiations "have again returned to a dead-end situation for us."

Making his comments, Putin frequently seemed to ramble or stammer. Only occasionally did he adopt the icy, confident demeanour that has been his trademark over more than 22 years as Russia's leader.

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.

Putin ally detained

Zelenskiy told Russia to release prisoners of war if it wants the Kremlin's most high-profile political ally in the country freed.

Ukraine said that Viktor Medvedchuk, the leader of the Opposition Platform - For Life party, had been recaptured. In February, the authorities said he had escaped house arrest after a treason case was opened.

The politician who says Putin is godfather to his daughter has denied wrongdoing. A spokesperson was not immediately available for comment.

"I propose to the Russian Federation: exchange this guy of yours for our guys and girls now held in Russian captivity," Zelenskiy said in his address.

Alongside a photo of Medvedchuk in handcuffs, the head of Ukraine's security service Ivan Bakanov said on Facebook that operatives "conducted this lightning-fast and dangerous multi-level special operation" to arrest him.

A Kremlin spokesperson was cited by the Tass news agency as saying he had seen the photo and could not say whether it was genuine.

Russia says it now aims to capture more territory on behalf of separatists in two eastern provinces, known as the Donbas, which includes the besieged port of Mariupol.

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

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.

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

The United States and Britain have said they were trying to verify the reports. The Organisation for the Prohibition of Chemical Weapons (OPCW) said it was closely monitoring the situation.

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.

The United States is expected to announce $750 million more in military assistance, two officials told Reuters, likely including heavy ground artillery systems to Ukraine, including howitzers, in a sign the war is expected to drag on.