A tribute to Slobodan Praljak in Zagreb on November 30, 2017. Photo: AFP

Bosnian Croat war criminal Slobodan Praljak likely died from heart failure after swallowing potassium cyanide, Dutch prosecutors said Friday, two days after he committed suicide in front of UN judges.

"The preliminary results of the toxicological test showed that Mister Praljak had a concentration of potassium cyanide in his blood," the Dutch prosecution said in a statement in English.

"This has resulted in a failure of the heart, which is pointed out as the suspected cause of death."

The preliminary results were released after an autopsy was carried out on the body of Praljak, who on Wednesday in a last act of defiance against the International Criminal Tribunal for the former Yugoslavia, swallowed the poison in front of UN judges just moments after they upheld his 20-year jail term.

"Praljak passed away in Westeinde hospital in The Hague after consuming a fluid substance in the courtroom of the International Criminal Tribunal for the former Yugoslavia (ICTY)," the Dutch prosecution service said in a statement.

Two Croatian experts were also present during Friday's autopsy carried out at the Netherlands Forensics Institute in The Hague.

Contacted by AFP prosecutor Marilyn Fikenscher confirmed that the autopsy was over, but said "we are still awaiting the final results."

She also said she could not go into details about what levels of potassium cyanide were found in Praljak's blood, nor whether his body remained in the National Foresenics Institute in The Hague where the autopsy was carried out.

"These are just preliminary results, we must wait until the final results," she said, "it doesn't happen a lot that someone commits suicide like this".