The first Bosnian Serb to plead guilty to taking part in the Srebrenica massacre has had his 27-year sentence reduced on appeal.
Nikolic has given the first glimpse into Srebrenica
Judges at the war crimes tribunal in The Hague cut the time Momir Nikolic will serve to 20 years.
Nikolic, an ex-intelligence officer in the Bosnian Serb army, made a plea bargain with prosecutors before his 2003 trial.
Nearly 8,000 Muslims men and boys were killed in the 1995 massacre.
Nikolic testified against a former superior and will do so again at another trial.
Nine former Bosnian Serb officers accused of complicity in the killings are still facing trial.
Judges at Nikolic's trial had refused to accept the plea bargain, saying he had played a pro-active role of significant importance to the unfolding massacre.
But the appeals court has now ruled that the judges made a number of errors in judgement, including not giving him enough credit for his co-operation with prosecutors.
UN safe haven
Nikolic, 51, was a deputy intelligence chief with the Bratunac brigade, which allegedly took part in the mass executions of civilians from the Muslim enclave of Srebrenica in 1995.
He was the first former Bosnian Serb officer to give an insight into the large-scale operation at Srebrenica, in eastern Bosnia-Hercegovina.
The mass executions are said to have been ordered by fugitive Bosnian Serb leader Radovan Karadzic.
The enclave was a designated UN safe haven for Bosnian Muslims, but after it was taken over by Serb forces thousands of Muslim men and boys were taken away and shot.
Srebrenica was under the protection of Dutch UN peacekeepers when it fell to the hands of the Serbs.