A Bosnian Serb general convicted by United Nations judges of genocide in the 1995 Srebrenica massacre, and described as the "right hand" of Ratko Mladic, has died in his cell, the UN said.
"Zdravko Tolimir passed away last night in the UN Detention Unit in [the Hague]," the UN said in a statement on Tuesday.
An investigation to establish the cause of death is now under way, the statement added, saying "the medical officer was called immediately and the Dutch authorities have commenced standard investigations as mandated under Dutch national law."
Once considered the right-hand man of Bosnian Serb army chief Ratko Mladic, Tolimir, 67, was sentenced to life in 2012 for crimes committed "on a massive scale" during the 1990s Bosnian war.
Tolimir's wife Nada Tolimir told Serbia's Kurir newspaper that she learned of his death late on Monday.
"He was ill, but the idiots did not allow him to come here and get treatment at home. I was surprised by the news," the former general's wife was quoted as saying.
In April, the International Criminal Tribunal for the former Yugoslavia appeals chamber upheld Tolimir's life sentence for genocide and crimes against humanity and war crimes.
Trial judges described Tolimir as Mladic's "eyes and ears", particularly in the July 1995 massacre at the supposedly UN-protected enclave of Srebrenica, where Bosnian Serb forces slaughtered almost 8,000 mostly Bosnian Muslim men and boys.
The incident was Europe's worst massacre since those committed during World War II.
Mladic himself is on trial at The Hague facing charges including genocide, war crimes and crimes against humanity, including for Srebrenica.
Tolimir was one of the most senior Bosnian Serbs to have a verdict handed down by the UN war crimes court and one of a handful of defendants found guilty of genocide.
He was, however, not the first top official to die while in the tribunal's custody.
Former Serbian strongman Slobodan Milosevic died in 2006 also while on trial for genocide, war crimes and crimes against humanity.
Source: Al Jazeera and agencies