The wartime commander of Bosnian Croat forces, Slobodan Praljak, died after he drank poison seconds after United Nations judges turned down his appeal against a 20-year sentence for war crimes against Bosnian Muslims.

Croatian state television reported that Praljak died in a hospital in The Hague this afternoon.

Appeals judges at the UN's Yugoslav war crimes tribunal upheld the convictions of six Bosnian Croats found guilty of war crimes and crimes against humanity during the 1990s, in the court's last verdict before it closes next month.

After the judge confirmed his sentence, Praljak, 72, took a quick drink from a glass.

He also said: "I am not a war criminal. I oppose this conviction."

The presiding judge suspended the hearing and called for a doctor.

An ambulance was at the building and paramedics went to the courtroom.

"Former head of the chief headquarters of the Croatian Defence Council, General Slobodan Praljak, died in a hospital in The Hague after he drank poison in a courtroom after the International Criminal Tribunal for the Former Yugoslavia confirmed his 20-year sentence for war crimes," Croatian TV said.

Appeals judges also upheld the convictions of five other Bosnian Croats found guilty of war crimes and crimes against humanity during the 1990s.

The former high-level politicians and defence officials were convicted in 2013 of participating in an "ethnic cleansing" campaign against Bosnian Muslims.

They include Jadranko Prlic, a former defence minister, whose 25-year jail sentence was upheld.

He had been found guilty of being part of a criminal enterprise by the wartime Croatian government of late President Franjo Tudjman, to create an ethnically pure state.

Several convictions for specific crimes were reversed in the appeal, but president Judge Carmel Agius said: "All six remain convicted of numerous and very serious crimes."