Russia committing genocide in Ukraine: Biden

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.

Biden used the term genocide, a significant escalation of the president's rhetoric, in a speech at an ethanol plant in Iowa and later stood by the description as he prepared to board Air Force One.

"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 on Tuesday.

"We'll let the lawyers decide internationally whether or not it qualifies, but it sure seems that way to me."

Biden has repeatedly called Putin a war criminal, but Tuesday was the first time he had accused Russia of genocide.

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

Many of the 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.

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

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 led to Russia's near total isolation 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-and-off peace negotiations "have again returned to a dead-end situation for us."

During 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.