The war crimes tribunal in The Hague has heard allegations that former Bosnian Serb leader Radovan Karadzic ordered the 1995 Srebrenica massacre.
Karadzic has been on the run since the end of the war
A former Bosnian Serb politician, Miroslav Deronjic, said Mr Karadzic told him days before the massacre that all local Muslims needed to be killed.
Mr Deronjic was testifying at a pre-appeal hearing for General Radislav Krstic, who was partly responsible.
The tribunal has charged Mr Karadzic with genocide over Srebrenica.
Mr Deronjic confessed to war crimes.
He was a civilian head of the Bratunac municipality near Srebrenica when the Bosnian Serb's political leader
Mr Karadzic told him on 9 July, 1995: "Miroslav, all of them need to be killed. Whatever you can lay your hands on," he told the hearing.
On 11 July, the Bosnian Serb army overran the UN-declared "safe area" of Srebrenica in eastern Bosnia.
Its troops separated the men from the women and children, taking them away to be executed over the ensuing week.
Alongside Mr Karadzic, the tribunal has also charged his military commander, Ratko Mladic, with genocide at Srebrenica Both men remain at large.
Witnesses who spoke at Friday's hearing said the slaughter of more than 7,000 Muslims at Srebrenica was planned at the highest level of the Bosnian Serb Government.
One witness - Lieutenant Colonel Dragan Obrenovic, commander of a brigade that reported to General Krstic - admitted his brigade participated in the slaughter after being given the order by General Mladic.
He told the hearing he had asked Mr Krstic why the killings had taken place.
"Krstic cut me short and said that we would speak no more about this," he said.
Experts say the statement he gave is significant because a commander can be convicted of genocide if he knew his troops were committing atrocities but failed to stop them.
General Krstic's defence has until now rested on a claim that prosecutors had never shown "whether there was an organised plan or policy... to destroy the Bosnian Muslims".
Prosecutor Norman Farrell said the new joint evidence was sufficient to prove Krstic, 58, - then commander of the Drina Corps - had personally overseen the killings.
Krstic is appealing against his conviction and the 46-year sentence handed down by the UN tribunal in August 2001 for genocide.
The prosecution is seeking to increase his sentence to life.