The UN has ordered an investigation after Bosnian Serb ex-leader Radovan Karadzic used his limited telephone access to take part in a public event in Montenegro from inside prison.
He gave a speech to people attending the event in Podgorica on Friday.
Karadzic was found guilty of genocide and war crimes by a UN tribunal in The Hague and was given a life sentence.
He was accused of a leading role in the killing of thousands of Muslims in Bosnia during the 1990s Balkan War.
Olufemi Elias, the UN court’s registrar confirmed that Karadzic had taken part in the event.
He said he had now ordered that the detention unit’s commander “listens to and summarises” Karadzic’s non-privileged phone calls for the past five days.