The US defence department has accused Russian President Vladimir Putin of acting with “depravity” in his invasion of Ukraine.
Spokesman John Kirby became visibly emotional as he asked how anyone “moral” could justify the atrocities committed by Russia.
But Russia’s ambassador to the US Anatoly Antonov has rejected the accusation.
He described Mr Kirby’s comment as “offensive and unacceptable”.
Ukrainian President Volodymyr Zelensky said on Friday he was still open to peace talks with Mr Putin.
But he said there was a high risk they could collapse amid Russian aggression.
Speaking to Polish media, Mr Zelensky said he wanted to meet Mr Putin because “a single man decides everything” in Russia.
But the destruction left by Russian forces in occupied areas has made any discussions tenuous, he said. “After Bucha and Mariupol people just want to kill them. When there is such attitude, it is hard to talk about anything.”
On Thursday, Ukraine announced a hunt for 10 Russian soldiers accused of war crimes in Bucha – a suburb north of Kyiv where at least 400 civilians were killed.
“I don’t think we fully appreciated the degree to which [Mr Putin] would visit that kind of violence and cruelty,” Mr Kirby said on Friday.
He dismissed Mr Putin’s stated justifications for the invasion – that he is protecting Russians and Ukraine from Nazism – adding: “It’s hard to square that rhetoric by what he’s actually doing inside Ukraine to innocent people, shot in the back of the head, hands tied behind their backs, pregnant women being killed, hospitals being bombed.”
But Mr Antonov accused Kirby of “resorting to street insults”.
“It has become a norm here that administration officials base their judgments on dirty lies of the Ukrainian authorities,” the Russian ambassador added.
The BBC’s Joel Gunter in Kyiv said there is growing evidence that Russia has forcibly deported large numbers of civilians across the border since it invaded the country in February.
Russian Foreign Minister Sergei Lavrov said in an interview published on Saturday that more than one million people have been evacuated from Ukraine to Russia since the war began in February.
Mr Lavrov told China’s state news agency Xinhua that it included some 120,000 foreigners, in addition to hundreds of thousands of people from the Russian-backed breakaway regions of eastern Ukraine – Donetsk and Luhansk.
Ukraine’s deputy prime minister told the BBC that Moscow has attempted to trade Ukrainian civilians for Russian military prisoners – a move forbidden by the Geneva Convention.
Russia confirmed on Friday that its military had conducted an air strike in Ukraine’s capital during a visit by the UN’s secretary general. Journalist Vira Hyrch was killed in the attack – the first of its kind in Kyiv in nearly two weeks – after the residential building she lived in was hit by a missile.
Russia’s defence ministry said it had deployed “high-precision, long-range air-based weapons” to target a missile factory in Kyiv.
More US aid for Ukraine
Now more than two months into the war, US President Joe Biden has asked Congress for $33bn (£27bn) in military, economic and humanitarian aid for Ukraine – a major ramp-up of American support for Ukraine.
The bumper package has been held up by congressional infighting over US domestic priorities, but US Speaker of the House Nancy Pelosi said on Friday she hoped to get it passed “as soon as possible”.
In Kyiv, Mr Zelensky called the bill’s passage “concrete proof” that freedom can defend itself against oppression.
“I am sure now that the Lend-Lease will help Ukraine and the whole free world to beat the ideological successors of the Nazis, who started a war against us,” he said in a late night address on Friday.
And the UK also said it was deploying some 8,000 troops for exercises across eastern Europe in a show of the Western world’s resolve against Russia’s continued aggression.