Former Bosnian Serb commander Ratko Mladic is due to address judges in the Hague as part of an appeal against his conviction for genocide and crimes against humanity.
He was jailed for life in 2017 for his part in the massacre at Srebrenica in 1995 when about 8,000 Muslim men and boys were killed.
Mladic’s lawyers have argued he was far away from the town when it happened.
The hearing was delayed by his health problems and coronavirus restrictions.
The second and final day of the hearing opened on Wednesday.
Meanwhile, the prosecution is urging judges to convict Mladic on a further genocide charge.
The Srebrenica massacre was the worst atrocity in Europe since World War Two.
How did the first day of the hearing pass?
Mladic, 78, appeared in court on Tuesday, briefly wearing a surgical mask before removing it.
His lawyers told the UN court that the proceedings should not go ahead until a medical team had reviewed his capacity to take part.
They argued he had been wrongly convicted of “unscheduled incidents” made as accusations during his trial.
Originally convicted on 10 counts, prosecutors say he should also be found guilty of genocide against Bosniaks and Croats in 1992.
The trial appeared to be affected by technical issues.
Presiding Judge Prisca Matimba Nyambe, who was among the judges following proceedings by video link, said at one point she was unable to decipher the defence lawyer’s words and would have to rely on transcripts.
At another point, defence lawyer Dragan Ivetic complained he could not communicate with his client “or be assured that he is able to meaningfully follow proceedings”.
The “Butcher of Bosnia” earlier needed an operation to remove a benign polyp on his colon, and had a request for a delay on health grounds rejected ahead of the hearing.
What was Mladic convicted of?
Mladic was the military commander of Bosnian Serb forces against Bosnian Croat and Bosniak armies. He went on trial at the International Criminal Tribunal for the former Yugoslavia (ICTY) in 2012, and was convicted in 2017.
The court found he had “significantly contributed” to the genocide at Srebrenica.
The other charges included war crimes and crimes against humanity.
He was cleared of a second count of genocide in other municipalities. The court will hear an appeal by prosecutors against this acquittal this week.
The Mothers of Srebrenica, a group of women related to victims of a massacre in the town in 1995, said the tribunal “must not lose motivation, and must carry out its mission”.
“We hope Mladic will be found guilty for genocide in other towns as well,” Munira Subasic, the organisation’s president, told AFP.
At the end of the war in 1995 Mladic went into hiding and lived in obscurity in Serbia, protected by family and elements of the security forces.
He was finally tracked down and arrested at a cousin’s house in rural northern Serbia in 2011 after 16 years on the run.