Bosnian Croat war crimes convict Slobodan Praljak used cyanide to kill himself in court after losing his appeal, according to preliminary post mortem findings, Dutch prosecutors have said.

Praljak, 72, announced that he had taken poison immediately after his conviction and 20-year sentence were upheld by appeals judges at the International Criminal Tribunal on Wednesday, and died shortly afterward.

In a statement, prosecutors said a toxicological test had found that Praljak "had a concentration of potassium cyanide in his blood".

"This resulted in a failure of the heart, which is indicated as the suspected cause of death," they added.

Earlier on Friday, the tribunal said it had launched its own independent review into the death.

Praljak committed suicide in the final minutes of the last verdict at the court before it closes later this month.

"I just drank poison," the ex-general told the stunned court in a hearing that was being broadcast online.

"I am not a war criminal. I oppose this conviction," he said.

Prosecutors said they were focusing their investigation on how Praljak had managed to obtain a banned substance in the high-security UN building.

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".