(Reuters) - U.S. 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 U.S. 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 denied targeting civilians and has said Ukrainian and Western allegations of war crimes are fabricated.

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 General Headquarters of the Armed Forces of Ukraine said in a Wednesday briefing that Russian forces were maintaining rocket and bomb attacks on civilian infrastructure in the Kharkiv region in the northeast and the Zaporizhzhia region in central Ukraine. Reuters could not verify the reports.

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.

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.

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."

The Russian leader frequently seemed to ramble or stammer. Only occasionally did he adopt the icy, confident demeanour that has been his trademark fopr 22 years in power.

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.