Slobodan Praljak yelled, "I am not a war criminal!" and appeared to drink from a small bottle, seconds after judges reconfirmed his 20-year prison sentence for involvement in a campaign to drive Muslims out of a would-be Bosnian Croat ministate in Bosnia in the early 1990s.
Presiding Judge Carmel Agius had overturned some of Praljak's convictions but left his sentence unchanged.
Agius quickly halted the hearing at the International Criminal Tribunal for the former Yugoslavia. It could not immediately be confirmed whether Praljak had taken poison or the status of his health.
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.
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The appeals judges upheld a key finding that late Croat President Franjo Tudjman was a member of a plan to create a Croat mini-state in Bosnia.
Two other suspects had also had their sentences upheld before the hearing was halted.
The court's press office declined comment.
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