A Bosnian Serb general convicted by United Nations judges of genocide in the 1995 Srebrenica massacre has died in his cell, the Yugoslav war crimes tribunal has announced.

Zdravko Tolimir, 67, died on Monday night at the court's detention unit in The Hague, the tribunal said in a statement.

The cause of death was not announced. The court said that local authorities "have commenced standard investigations as mandated under Dutch national law".

The organisation dealing with legacy issues from the Yugoslav and Rwanda war crimes tribunals as they wind down their operations also announced an inquiry.

Tolimir, the Bosnian Serb Army's top intelligence officer, was convicted in December 2012 of genocide and other crimes in the massacre by Bosnian Serb forces of some 8,000 Muslim men in Srebrenica, eastern Bosnia - Europe's worst mass killing since World War II. He was sentenced to life imprisonment.

Announcing the verdicts, Judge Christoph Fluegge said witnesses described Tolimir as Bosnian Serb military chief Mladic's "right hand. His eyes and ears".

The judge said Tolimir had "full knowledge of the despicable criminal operations" of Bosnian Serb forces that carried out the massacre.

Appeals judges upheld most of his convictions last April and confirmed his sentence.

Despite the genocide conviction, Tolimir was considered a hero among Bosnian Serbs.

In a statement, Bosnian Serb Prime Minister Zeljka Cvijanovic called him, "an exceptional man, brave and honourable general who ... contributed greatly to the defence of his people" and the Serb-controlled part of Bosnia.

The alleged main architects of Bosnian Serb atrocities, Mladic and former Bosnian Serb leader Radovan Karadzic, are both being held at the tribunal's detention unit where Tolimir died.

Mladic's trial is still under way, while verdicts in Karadzic's case are expected to be delivered early this year.

Both men are charged with genocide and other crimes allegedly committed during the 1992-95 Bosnian war that left 100,000 people dead.