UN judges postponed Wednesday an appeals hearing in the case of Bosnian Serb ex-military chief Ratko Mladic, who had to undergo an operation days before the case was due.
Mladic, once dubbed the Butcher of Bosnia was handed life behind bars in 2017 for his role in the country's bloody 1990 civil war.
This included for genocide committed by his Bosnian Serb forces in the small eastern Bosnian town of Srebrenica in mid-1995, Europe's worst bloodshed since World War II.
About 100,000 people were killed and 2.2 million others displaced in the 1992-95 war, which erupted as communal rivalries tore Yugoslavia apart after the fall of communism.
The International Residual Mechanism for Criminal Tribunal said it "vacates the scheduling order and stays the appeals hearing until further notice on the basis of Mladic's upcoming surgery".
Hearings were due to start next Tuesday.
Doctors at a high-security detention unit in The Hague where Mladic has been in custody since 2011 said the 76-year-old was undergoing an operation "to remove a benign polyp from his colon".
The judges said they wanted to be updated weekly on Mladic's condition "to facilitate the expeditious rescheduling of an appeals hearing to a date approximately six weeks after the surgery."
In one of its final judgments, the International Criminal Tribunal for the former Yugoslavia (ICTY) acquitted the brazen ex-commander of genocide in certain municipalities, a fact which now forms the bulk of the prosecution's appeal.
Mladic's crimes however were "amongst the most heinous known to humankind", the judge said when handing down the sentence.
At Srebrenica, Bosnian Serb forces overran UN peacekeepers before slaughtering almost 8,000 Muslim men and boys and dumping their bodies into mass graves.
Mladic was among the top leaders to face international justice over the Balkans wars -- along with former Bosnian Serb leader Radovan Karadzic and ex-Yugoslav president Slobodan Milosevic.
Milosevic died in his cell in The Hague in March 2006, suffering a heart attack before his trial had finished.
Karadzic was convicted of genocide in 2016 for the Srebrenica massacre and other atrocities during the war and sentenced to 40 years.
After an appeal, judges increased his sentence to life, saying the initial term had underestimated the "sheer scale and systematic cruelty" of his crimes.