Bosnian Croat wartime general Slobodan Praljak has killed himself by drinking poison, after a United Nations court in The Hague upheld a previous ruling for his war crimes against Bosnian Muslims.

The 72-year-old former general told shocked judges in the International Criminal Tribunal for the former Yugoslavia (ICTY) on Wednesday that he "drank poison" after they blocked his appeal for a 20-year-old sentence.

"I just drank poison, I am not a war criminal. I oppose this conviction" he said after taking a swing from a small bottle in the full view of cameras.

A lawyer who was in the courtroom at the time said Praljak then sat back down and slumped in his chair as court officials surrounded him.

Carmel Agius, the presiding judge who was handing down Praljak's sentence, suspended the court and allowed paramedics to take him to a hospital. He was pronounced dead after an hour.

UN officials confirmed the former general's death after it was initially reported by Croatian media. Dutch police declared the courtroom to be a crime scene.

Praljak was among the six remaining former Bosnian Croat political and military officials linked to the wartime Croatian government of late President Franjo Tudjman to stand trial at the ICTY before the court's final verdict.

Established by the UN in 1993, the ICTY will be shuttered after its mandate expires next month.

The judges had charged Praljak with ordering the destruction of a 16th-century bridge in the city of Mostar in southern Bosnia and Herzegovina. In the first trial, judges had said that the November 1993 order “caused disproportionate damage to the Muslim civilian population.”

The city underwent some of the worst of the most intense clashes during the war, with nearly 80 percent of its eastern parts destroyed.

The ruling on Wednesday accepted part of Praljak’s arguments that the bridge had been a legitimate military target during the conflict. The judges also overturned some of his convictions after the appeal but refused to reduce the jail sentence.

The court's lead suspect, former Yugoslav President Slobodan Milosevic, died after a heart attack in March 2006, months before a ruling for his genocide case.

Two more defendants awaiting trial at the court have also committed suicide, according to court documents. Slavko Dogmanovic and Milan Babić were found dead in their cells in 1998 and 2006 respectively, both hanging themselves.