The chief prosecutor of the United Nations war crimes tribunal has urged Serbia to step up efforts to locate four leading fugitive war crimes suspects.

Serge Brammertz told the United Nations Security Council he believes that Radovan Karadzic, Ratko Mladic, Stojan Zupljanin and Goran Hadzic are within reach of Serbian authorities and can be arrested.

The prosecutor, who replaced Carla del Ponte earlier this year, reported no improvement in Serbian cooperation with the court over the past six months.

Brammertz also pressed authorities in Bosnia-Herzegovina to adopt a more pro-active approach against those helping the fugitives avoid capture. He also called on Croatia to comply immediately with a number of requests for key documents required for ongoing trials.

The prosecutor also stressed his belief that the tribunal should continue its work until the trials of all war crimes suspects are completed.

The tribunal indicted Karadzic and Mladic for their roles in attacks on civilians during the Bosnian Conflict.

Zupljanin faces charges of crimes against Croats and Muslims in western Bosnia.

Hadzic is charged with running a "joint criminal enterprise" aimed at expelling ethnic Croats from Serb-held areas of Croatia.

Some information for this report was provided by AFP, AP and Reuters.