They were convicted for their involvement in a campaign to drive Muslims out of a would-be Bosnian Croat ministate in Bosnia in the early 1990s.
A United Nations war crimes appeal trial was suspended on Wednesday after a former Croatian general claimed to have drank poison upon being sentenced to 20 years in jail.
A wartime commander of Bosnian Croat forces has died after poisoning himself on live television in the International Criminal Court for the former Yugoslavia (ICTY) in The Hague. Slobodan Praljak, 72, shouted a declaration that he is not a war criminal before downing an unknown substance from a small bottle.
The hearing was suspended after one of the suspects, former military commander Slobodan Praljak, drank from a bottle in court and claimed to have ingested poison.
Croatian Prime Minister Andrej Plenkovic later confirmed that Praljak had died, saying in a government tweet, "On behalf of the Government of the Republic of Croatia and on my own behalf, I want to express my deepest condolences to the family of General Slobodan Praljak".More news: Daredevils jump from mountain top, land in plane mid-air
Tribunal spokesman Nenad Golcevski, when asked by AP if he could confirm the death, said: "I have no information to share at this point".
Several convictions for specific crimes for Prlic and the other defendants were reversed in an appeal, but the presiding judge said "all six remain convicted of numerous and very serious crimes".
Some of his convictions for actions in the later years of the conflict were overturned, but his sentence was not. He was originally sentenced to 20 years in jail in 2013.
An ambulance could later be seen arriving outside the tribunal while a helicopter hovered above the scene.
The groundbreaking ICTY is to finally close next month.