The ex-Bosnian Serbs' leader Radovan Karadzic and former military leader Ratko Mladic are appearing together at the war crimes tribunal in The Hague for the first time.

Karadzic, who is facing 11 charges, including genocide in Srebrenica, has called his ex-deputy Mladic to give evidence at the UN Yugoslav war crimes tribunal (ICTY), the BBC reports.

Karadzic is hoping that his former ally's testimony will support his claims that the orders to commit war crimes did not come from him.

In the 1995 Srebrenica massacre, Bosnian Serb forces overran the UN-defended safe area, subsequently killing more than 7,500 Muslims, in what was Europe's worst atrocity since World War II.

Ratko Mladic, who was the general in charge of the troops at the time, is also being tried at the ICTY. He was arrested in 2011 in northern Serbia, after 16 years on the run.