US President Joe Biden has accused Russia's forces of committing "genocide" in Ukraine, saying Vladimir Putin appears intent on "trying to wipe out the idea" of a distinct Ukrainian identity.

Ukrainian President Volodymyr Zelensky too has accused Russia of genocide over the discovery of hundreds of bodies of civilians in areas that were controlled by Russian forces and labelled the bloody siege of the southern port of Mariupol a "crime against humanity".

We look at the different categories of the most serious crimes known to man, which the International Criminal Court (ICC) in The Hague was set up to prosecute.

Neither Russia nor Ukraine are parties to the ICC but Kyiv has accepted the court's jurisdiction for crimes committed on its territory since Russia's invasion of Crimea in 2014.

The court opened a probe into suspected war crimes in Ukraine on March 3 this year.

War crimes are serious violations of international law against civilians and combatants during armed conflict.

The parameters of what constitutes a war crime are set out in Article 8 of the 1998 Rome Statute that established the ICC.

It defines them as "grave breaches" of the 1949 Geneva Conventions covering more than 50 scenarios, including killing, torture, rape and the taking of hostages as well as attacks on humanitarian missions.

Article 8 also covers deliberate attacks on civilians or "towns, villages, dwellings or buildings which are undefended and which are not military objectives" as well as the "deportation or transfer of all or parts of the population" of an occupied territory.

Ukraine's authorities say they have received 5,600 complaints of alleged war crimes by Russian forces since the invasion began on February 24.

Ukrainian President Volodymyr Zelensky has accused Russia of 'genocide'
Ukrainian President Volodymyr Zelensky has accused Russia of 'genocide' AFP / RONALDO SCHEMIDT

The notion of such a crime was first laid down on August 8, 1945, and codified in article 7 of the Rome Statute. It involves "a widespread or systematic attack directed against any civilian population" including "murder" and "extermination" as well as "enslavement" and "deportation or forcible transfer".

Crimes against humanity can occur in peace-time and include torture, rape and discrimination, be it racial, ethnic, cultural, religious or gender-based.

Genocide as a legal concept dating back to the Nuremburg trials of Nazi war criminals, with Polish-Jewish lawyer Raphael Lemkin coining the term to describe the Nazi extermination of six million Jews.

The crime of genocide was formally created in the Genocide Convention of 1948 to describe "acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group".

Genocide is a "very specific international crime" which is difficult to prove, says Cecily Rose, professor of international law at the University of Leiden in the Netherlands, noting that it demands proof of the "mental motivation" behind it.

The ICC added a crime of aggression to its remit in 2017 to include attacks on "the sovereignty, territorial integrity or political independence" of another country.

The offence aims to ensure that political and military leaders are held accountable for invasions, but it cannot be used against the dozens of ICC members that have not recognised the court's jurisdiction for the crime, nor against non-members.

Legal experts say bringing such a case against Russia's president may require the establishment of a special tribunal for Ukraine.