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.
Newcomer: crime of aggression
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 offense 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.