A United Nations tribunal said it was investigating how former Bosnian Serb leader Radovan Karadzic made a phone call to an event held in Montenegro from his jail cell in The Hague.
A tribunal spokesman said in a statement on May 7 that Karadzic's call was unauthorized.
"The commanding officer did not give the detainee prior approval to use the communication facilities," Olufemi Elias said. There are "reasonable grounds to believe that the detainee may have committed an offense.
The incident occurred two months after a panel of judges increased Karadzic's sentence for genocide and other crimes in the 1990s to life in prison.
Elias said the prison unit's commander was ordered to monitor Karadzic's non-privileged phone calls.
The May 3 event that Karadzic called into was organized by the Serbian National Council and the Serbian Information and Cultural Center in Podgorica.
The president of the Serbian National Council, Momcilo Vuksanovic, told RFE/RL there was nothing controversial in the call.
"He wanted to greet the participants of the forum and this was allowed. I do not see anything controversial in that," he said. "He is a Hague convict, but the court had double standards when war crimes were concerned and we see that now in the judgment for...Karadzic.”
Karadzic and his military chief, Ratko Mladic, were among the last suspects put on trial by the United Nations tribunal for their roles in Bosnia's 1992-95 civil war.
Tribunal officials ordered increased monitoring for Mladic after he spoke to a Serbian television station in November and said he was "sending kisses."
Mladic is currently appealing a life sentence on similar charges.