A UN tribunal has ordered an investigation after the former Bosnian Serb leader Radovan Karadžić allegedly took part in a public event in Montenegro via telephone from his prison cell in the Netherlands.

The incident came just two months after Karadžić had his sentence for genocide and other crimes in the 1990s increased to life behind bars by appeal court judges in The Hague.

“On the evening of 3 May, the detainee appeared to have participated in a public event (lecture/discussion) in Podgorica, Montenegro,” the UN court’s registrar Olufemi Elias said in a court document. “The commanding officer did not give the detainee prior approval to use the communication facilities,” Elias said, adding there were “reasonable grounds to believe that the detainee may have committed an offence”.

The registrar has ordered the detention unit’s commander to listen to and summarise Karadžić’s non-privileged phone calls for the past five days.

Karadžić’s lawyer Peter Robinson confirmed that his client joined a public discussion held in Podgorica on Saturday about political issues in the region, including the role of Nato, which intervened in the Bosnian war. “As I understand it, he urged tolerance,” Robinson said.

Karadžić is currently waiting to be transferred to a prison outside the Netherlands to serve his time for his role in Bosnia’s bloody 1992-1995 civil war, including the Srebrenica massacre.

In March, judges increased his sentence from 40 years to life, saying the initial jail term had underestimated the “sheer scale and systematic cruelty” of his crimes. Last month, they denied Karadžić a last ditch attempt to appeal against his life sentence.

Karadžić and his military chief Ratko Mladic were among the last suspects put on trial by the UN tribunal in The Hague for Bosnia’s bloody 1992-95 civil war. Mladic, who is currently appealing against a life sentence on similar charges, was last year placed under increased monitoring after speaking to a Serbian television station in November saying he was “sending kisses”.