A war crimes convict that swallowed a liquid during a dramatic court appearance died of cyanide poisoning, Dutch prosecutors say.

Slobodan Praljak took the poison while appearing before UN judges in The Hague after his conviction and 20-year sentence were upheld.

The 72-year-old was convicted in 2013 of crimes including murder, persecution and deportation for his role in the attempt to create a Bosnian Croat mini-state in Bosnia in the early 1990s during the break-up of Yugoslavia.

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

On Wednesday, the former Croatian army chief drank from a small bottle and yelled "I am not a war criminal, I oppose this conviction" as judges upheld his conviction.

The 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 later, with presiding judge Carmel Aguis saying: "Courtroom one is now a crime scene."

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

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.

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.