“Evidence is mounting,” he said on Tuesday, that Moscow is trying to “wipe out the idea of even being Ukrainian”.
While Ukrainian officials and supporters have used the term genocide, other international leaders and organisations have shied away from using the word.
Genocide, they point out, has a specific legal definition and carries severe implications.
Here is everything you need to know about the term and what Mr Biden’s comments could mean for Russia.
What is the definition of genocide?
The term genocide was coined by Jewish-Polish lawyer Raphael Lemkin, who studied linguistics in the now Ukrainian city of Lviv.
Disturbed that there was no specific crime for targeting an ethnic, racial or religious group with extermination solely on the basis of its identity, he made it his life’s work to have it defined as a crime in international law.
He wrote in 1948 that the use of the term genocide “is intended rather to signify a co-ordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves”.
Lemkin, who lost family members in the Holocaust, worked on the Nuremberg trials of Nazi war criminals. His work paid off.
After the Second World War, the crime of genocide gained international recognition with the 1948 Convention on the Prevention and Punishment of the Crime of Genocide, or the Genocide Convention, for short.
The 1948 Genocide Convention defines genocide as a crime committed “with intent to destroy, in whole or in part, a national, ethnic, racial or religious group”.
Criminal acts comprising genocide include killing members of the group, causing them serious bodily or mental harm, creating conditions calculated to destroy them, preventing births or forcibly transferring children to other groups.
The UN convention makes this illegal – but it’s far harder to prove.
The convention calls on all states that have ratified it to enact legislation to criminalise genocide. It states that anyone so accused should face trial in the country in which the action took place “or by such international penal tribunal as may have jurisdiction”.
Who has used the word genocide about Ukraine?
As well as Mr Biden, the Ukrainian government has called Russia’s offensive a genocide and has pressed the West to use the term. Russian President Vladimir Putin had previously accused Kyiv of committing genocide against Russian-speaking communities in Ukraine.
British Prime Minister Boris Johnson said last week that the scale of atrocities “doesn’t look far short of genocide”.
But Ukraine’s call for the wider adoption of the term has yet to be adopted by other international leaders.
UN Secretary General Antonio Guterres called Russia’s actions in Ukraine “a violation of international law” but has not used the word genocide.
While French President Emmanuel Macron has repeatedly condemned Russia’s actions, he said Mr Biden’s use of the term could be an unhelpful “verbal escalation”.
“I would say that Russia unilaterally unleashed the most brutal war, that it is now established that war crimes were committed by the Russian army,” he told France 2 TV. Yet, he said, it was best to be “careful” with terminology.
Who has a say on what constitutes genocide?
More or less anyone can brand an act of violence genocide but some accusations carry more weight than others.
Numerous instances of mass violence have been labelled genocide, by many countries and international bodies, since 1948. But only three cases since then met the threshold for the International Criminal Court in The Hague, although several charges filed have not yet been tried.
These are the Khmer Rouge’s slaughter of an estimated 1.7 million, including minority Cham Cambodians and Vietnamese, in the 1970s, the mass killings of 800,000 Tutsis in Rwanda in 1994 and the Srebrenica massacre of 8,000 Muslim men and boys in Bosnia the next year.
There have been convictions in all three cases.
The definition of genocide is quite specific and may not always be applicable, even to the mass killing of civilians.
“Genocide is a difficult crime to prove. Parties have to bring a lot to the table,” Melanie O’Brien, president of the International Association of Genocide Scholars, told Reuters.
She said it requires the prosecutors demonstrate intent, the targeting of a protected group and crimes such as the forcible removal of children.
Has there been a genocide in Ukraine?
That is not the official position of many countries but the International Criminal Court, which can try cases of genocide as well as crimes against humanity, opened an investigation into allegations of war crimes.
Ukrainian prosecutors have also been compiling what they say are hundreds of cases of possible war crimes.
But experts question whether there is evidence of genocide at this stage.
“It is not immediately apparent to me, on the basis of evidence available, that this standard is met,” said Philippe Sands, director of the Centre for International Courts and Tribunals at University College London.
“That said, there is genocidal rhetoric and it is important to be alert. Equally, it must be said, that there is no hierarchy of crimes in international law, and crimes against humanity, which appear to be occurring, are no less terrible than genocide,” he said.
This is an important point: even if a mass act of violence does not satisfy the legal definition of genocide it can still be a crime against humanity or a war crime which is also illegal under international law.
One of the key points that separate genocide from crimes against humanity is intent, which the UN says is the hardest to prove.
“Intent is the most difficult element to determine. To constitute genocide, there must be a proven intent on the part of perpetrators to physically destroy a national, ethnic, racial or religious group,” the UN said.
Rather than wrangle over whether genocide has been committed, Mr Sands pointed out there are other avenues to pursue in holding to account those who may have broken international law.
“I would also put the accent on the crime of aggression, waging a manifestly illegal war – it’s the only way to reach the top table with any degree of certitude.”
Mr Sands said Mr Biden’s remarks may not represent a legal accusation or even the US government’s official position.
“It may be that the term is being used in a political rather than legal sense.”
Eliav Lieblich, a professor of international criminal law at Tel Aviv University, picked up Mr Sands’ point that without a hierarchy in international law Ukraine could seek charges of crimes against humanity rather than genocide to avoid the wrangling over the definition.
“Charging Russians with crimes against humanity or war crimes is much easier legally since you don’t have to prove the special intent required for the crime of genocide, meaning, that the act was with intent to destroy a group as such,” Prof Lieblich said.
“There might be some marginal legal effects to classifying what’s happening in Ukraine as genocide, but clearly the key issue here is expressive. Genocide carries connotations of a supreme evil, beyond any tangible legal effects,” he said.
Sergey Vasiliev, an associate professor at the Amsterdam Center for Criminal Justice also highlights the political aspect of the term.
"In public discourse, genocide is often seen as the ‘crime of crimes’. The notion immediately evokes strong emotions and the imagery of past genocides such as the Holocaust, Srebrenica and the 1994 Rwandan genocide after which the international community made the solemn promise of ‘never again’ – broken over and over again," he says.
"Calling a situation in which mass atrocities are committed (such as what has come to surface in Bucha) as genocide often serves as a rhetorical trigger to spur the international community, especially its hesitating members, into more decisive action which may range from the harsher sanctions against the culprit state to joining as a party to the armed conflict."
Who tries cases of genocide?
As the 1944 convention states, cases should be tried in the country where the action took place. That’s not always possible and any signatory country can try a case as well as the International Criminal Court.
The case of the murder of tens of thousands of Yazidi and minority groups by ISIS in Iraq between 2014 and 2017 has been branded genocide by dozens of countries and UN bodies. The UN Security Council has formed an investigatory team to determine if the killings amount to genocide but has yet to instruct the International Criminal Court to take up the case.
That did not stop Germany last year from convicting Iraqi citizen and ISIS member Taha Al Jumailly, 29, to life in prison for genocide for his role in the killings.
Implications of genocide accusation for Russia
In the short term, the implications of Mr Biden’s remarks are likely to be limited.
There is no indication that the US plans to file charges against Russian leaders or military commanders or seek their extradition to face a trial.
So, unless more states start to adopt the position and a case is brought before the International Criminal Court, it is unclear that Mr Biden’s remarks will have any tangible legal impact on Russia.