War criminal Slobodan Praljak has died in hospital after drinking what he claimed to be poison during a court appearance, Croatia's prime minister has confirmed.

The former Croatian army chief drank from a small bottle and yelled "I am not a war criminal, I oppose this conviction" when his 20-year sentence was upheld by UN judges at The Hague.

The dramatic turn of events, which was broadcast on a video feed, forced the judges to temporarily suspend the hearing, with paramedics seen entering the courtroom.

The hearing resumed a few hours after the shocking development, with presiding judge Carmel Aguis saying: "Courtroom one is now a crime scene."

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Image: Praljak is one of six political and military appealing against convictions

Dutch police say they are investigating.

Praljak was convicted of involvement in a campaign to drive Muslims out of Bosnia and create an ethnically pure Croat state during the Bosnian war in the 1990s sparked by the break-up of Yugoslavia.

The conflict mainly saw Bosnian Muslims fighting Bosnian Serbs, but there were also deadly clashes involving Bosnian Muslims and Croats after an alliance fell apart.

A total of 100,000 people died and 2.2 million were displaced in the three-year war.

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Croatian PM Andrej Plenkovic said: "We have all unfortunately witnessed his act by which he took his own life."

He said Praljak's course of action reflects the "deep moral injustice" suffered by six Bosnian Croats who unsuccessfully appealed against their convictions at the International Criminal Tribunal for the former Yugoslavia (ICTY).

Some of Paljak's convictions were overturned, but the judge left his sentence unchanged.

Former war correspondent Martin Bell - who covered the Bosnian war and met Praljak on several occasions - told Sky News he was not surprised by his "theatrical" demise.

"What you have to understand about him is before he was ever a soldier he was a theatrical director - he ran one of the theatres in Belgrade," he said.

"They talk about the theatre of war for a reason. When I first met him it was on the front line in Croatia in 1991, and as soon as the camera crew arrived all hell erupted. There was a fantastic gun battle and when I later found out who he was I wondered if he actually stage managed it for the benefit of the cameras.

"He was a theatrical character and he died in a theatrical way."

Thousands died at the hands of Mladic

The six Bosnian Croats who unsuccessfully appealed includes former defence minister Jadranko Prlic, whose 25-year sentence for involvement in the campaign to create an ethnically pure state was upheld.

The court last week convicted former Bosnian Serb military chief Ratko Mladic of genocide.

Croatia claims it has "clean hands" over the 1992-95 war in Bosnia and wanted the decision overturned.

Zagreb has also expressed anger at the UN judges for upholding a finding that the late Croat president Franjo Tudman was part of a plan to create a ministate in Bosnia.

Tudjman's son, Miroslav, said Praljak's move was a "consequence of his moral position not to accept the verdict that has nothing to do with justice or reality".

The remaining three suspects - Milivoj Petkovic, 68, Valentin Coric, 61, and Berislav Pusic, 65 - had their sentences confirmed.

The court, established by the UN in 1993, is due to close when its mandate expires at the end of this year. It has indicted 161 suspects, of which 90 have been convicted.