The United Nations war crimes tribunal in The Hague has suspended an appeal case of a Bosnian Croat defendant after he apparently drank poison upon hearing his 20-year sentence was upheld.

Wartime commander of Bosnian Croat forces, Slobodan Praljak, drank from a small bottle or flask after declaring: "What I am drinking now is poison.

"I'm not a war criminal," Praljak shouted at the presiding judge before he sat back down and slumped in his chair.

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

A court guard said Praljak was still "being treated" more than an hour after he said he drank poison.

The tribunal on Wednesday was handing down its last judgment in an appeal by six Bosnian Croat political and military leaders, who were convicted in 2013 of persecuting, expelling and murdering Bosnian Muslims during the 1992-1995 war.

Speaking to Al Jazeera from Sarajevo, Denis Dzidic, deputy editor for the Detecor project, said policemen and an ambulance were ordered to the tribunal building.

"Slobodan Praljak had his first instance verdict confirmed, in which he was sentenced to 20 years in prison. He said that he did not accept the verdict, that he was not a war criminal and then drank the substance.

"The judge then paused the proceedings and ordered the glass [from which Praljak drank] not be taken from court. Now we are waiting to see what will happen. The verdict was mid-way and the first three defendants had their sentences confirmed; 25, 20, and, for Praljak also, 20 years in prison," Dzidic said.

Wednesday's hearing is the final case to be completed at the groundbreaking International Criminal Tribunal for the former Yugoslavia before it closes its doors next month.

The tribunal, which last week convicted former Bosnian Serb military chief General Ratko Mladic of genocide and other crimes, was set up in 1993, while fighting still raged in the former Yugoslavia. It indicted 161 suspects and convicted 90 of them.

The original conviction said that late Croat President Franjo Tudjman was a key member of a plan to create a Croat mini-state in Bosnia.