The US and Russian ambassadors clashed over Ukraine in a rare joint appearance in Beijing on Monday.
Nicholas Burns, the United States ambassador, told the World Peace Forum in Beijing that Russia's "unprovoked" invasion was "the greatest threat to the world order".
"Russia's war in Ukraine, the fact that Russia crossed the border with an armed force, unprovoked, and has started this war with so much human suffering, so many innocent civilians dead - this is a direct violation of the UN Charter," the ambassador told the event, which was hosted by Tsinghua University.
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He also warned that other countries were suffering because Russia's blockade of Ukraine's ports had disrupted grain exports.
"There's a food security crisis. It's been compounded by this illegal, unjust and brutal war of the Russian Federation against Ukraine," he said.
The two ambassadors shake hands before the event at the World Peace Forum. Photo: AP alt=The two ambassadors shake hands before the event at the World Peace Forum. Photo: AP>
But Andrey Denisov, the Russian ambassador, who spoke after Burns, said he "totally disagreed" with the comments and his American counterpart had ignored the backdrop of Nato expansion.
"Nato, in essence, is involved in the war with Russia through proxies," Denisov said.
He also insisted: "There is no blockade [of grain exports]. All the corridors are open. The latest step by Russia is withdrawing troops from Snake Island."
Snake Island, a critical outpost for controlling Black Sea shipping lanes, was abandoned by Russian forces last week in what Moscow said was a gesture of goodwill. Ukraine said the Russians had been forced to retreat under a heavy barrage of artillery and missile fire.
Denisov praised Beijing for its "reasonable and balanced" approach, adding: "Basically, our colleagues here in China say that they clearly know where the roots of the Ukraine crisis are."
China has so far refused to condemn Russia's invasion and has criticised the imposition of sanctions and Nato's eastward expansion, saying the alliance has created more conflict rather than resolving problems.
Densiov said China had been calling on all sides to take a constructive position, and suggested it may be able to "play a role" as mediator.
"China has a good relationship with Ukraine. And that's where I hope that in one way, China can send some kind of signal to our neighbours to be more realistic," he said.