The Ukrainian government and a former British prime minister are pushing for a special criminal tribunal to prosecute Russian President Vladimir Putin and his allies over the invasion of Ukraine.
Former Prime Minister Gordon Brown said the call for a body to investigate the "crime of aggression" was based on the tribunals that prosecuted senior Nazis after World War II.
The Netherlands-based International Criminal Court is already investigating allegations that Russia has committed war crimes in Ukraine. But while it can investigate genocide, crimes against humanity and war crimes, Russia has not signed up to a separate ICC statute under which nations pledge not to commit "crimes of aggression."
Brown said that "this act of aggression by Russia ... cannot go uninvestigated, unprosecuted and unpunished."
"Putin must not be able to escape justice," he said.
Ukrainian Foreign Minister Dmytro Kuleba welcomed the call for a tribunal, which is backed by legal experts and academics from around the world, reports AP.
"We are fighting against an enemy who is much stronger than us. But international law is on our side," Kuleba told a meeting in London by video link from Ukraine.