A prominent ally of the former Yugoslav President, Slobodan Milosevic, and three of his aides have made their first appearance in court in Belgrade on charges of revealing state secrets.

The former head of Serbia's secret police, Radomir Markovic, who has been in custody since February, is also accused of destroying information from the police database after Mr Milosevic's downfall in October.

The prosecutor, Sinisa Simic, announced he had crucial evidence on computer diskettes to prove his case.

He said the diskettes had been copied by Mr Markovic and his officers to remove data them from police records.

Mr Markovic denies the charges, which also include his alleged involvement in political assassinations.

The panel of judges decided to conduct the trial behind closed doors because of what they said was a high risk of revealing state secrets to the public.