Mr Biden’s remarks were immediately welcomed by Volodymyr Zelensky, the Ukrainian president, who has already described the Russian invasion as genocidal.

Mr Zelensky’s rationale is simple.

Genocide – as defined in the wake of the Holocaust – is the crime of attempting to destroy, in whole or in part, a national or ethnic group.

Combined with Vladimir Putin’s remarks that Ukraine has no legitimacy as a state, crimes like the killings and rapes in the Kyiv suburb of Bucha look like part of a plan to destroy Ukraine as both a country and an identity.

Therefore, says Mr Zelensky, Russia is committing genocide.

Simon Schama, the British historian, agrees. “Raphael Lemkin coined genocide in 1944 as ‘the destruction of a nation or racial group’,” he wrote on Twitter on Wednesday, referring to the Polish lawyer who coined the term. “That is exactly Putin’s aim in Ukraine, so Biden’s use of the term is wholly justified.”

Ukraine submitted an application to the International Criminal Court to investigate planned acts of genocide by Russia early in March.

But others are more cautious.

Emmanuel Macron, the president of France, said on Wednesday that Russia had clearly committed war crimes but declined to use the word genocide, stating: “I’m not sure the escalation of words is helping the cause right now.”

A British official said it is Her Majesty’s Government’s long standing position that the crime of genocide should be judged by a competent court – not by national governments.

And a report from the Organization for Security and Co-operation in Europe (OSCE) published on the same day found “clear patterns of International Humanitarian Law (IHL) violations by the Russian forces in their conduct of hostilities,” including extra-judicial killings, sexual violence, intentional targeting of hospitals and other medical facilities, and use of human shields – but also made no mention of genocide.

That is partly a legal issue. The charge of genocide requires a very large burden of proof. It may also reflect a feeling that the scale of atrocity has not reached that of the horror in Rwanda.

And as far as we know, Russian forces have not systematically executed thousands of men and boys in the space of a few days, as the Republika Srpska did at Srebrenica.

There could also be political considerations.