A UN war crimes court said Friday it is launching an "independent expert review" into the death of a Bosnian Croat war criminal who appeared to drink poison in front of shocked judges.

The probe will complement the Dutch investigation into the death of Slobodan Praljak, the International Criminal Tribunal for the former Yugoslavia (ICTY) said in a statement.

Praljak, 72, died in hospital after drinking from a small brown glass bottle in the ICTY courtroom on Wednesday.

His act of defiance was broadcast live around the world and came moments after judges rejected his appeal, upholding his 20-year jail term for atrocities committed in a breakaway Bosnian Croat statelet during the 1990s wars.

The shocking images drew the curtain on two decades of work at the court, set up in 1993 to try those responsible for the worst atrocities in Europe since World War II.

But it remains a mystery exactly what the former theatre and movie director, known for his forcible courtroom presence and outbursts, drank and how he managed to evade tight security to smuggle it into the tribunal.

Dutch prosecutors are already investigating security lapses over the incident.

The prosecutors have said "for the time being the inquiry will focus on assisted suicide and violation of the Medicines Act."

The ICTY review, which will begin next week, will be led by Justice Hassan B. Jallow, Chief Justice of The Gambia and the former prosecutor of the International Criminal Tribunal for Rwanda.

The court "this morning initiated an independent expert review focusing on the ICTY internal operations, in accordance with standard procedures", the statement said.

It aims to provide a report before the end of the year, which will be made public "subject to due process and confidentiality considerations."