Serbia's war crimes prosecutor says people helping to protect fugitives Radovan Karadzic and Ratko Mladic have been identified.
Ratko Mladic and Radovan Karadzic (right): On the run for 10 years
Vladimir Vukcevic warned they faced prosecution if they did not stop aiding the ex-Bosnian Serb leaders - both wanted on genocide charges.
The comments are some of the strongest yet made by senior officials in Serbia.
In the past, Serbian officials have been accused of doing too little to apprehend war crimes fugitives.
UN war crimes prosecutor Carla del Ponte has urged Belgrade to arrest the two by the end of this month.
Mr Vukcevic did not name the individuals suspected of helping Mr Karadzic and Mr Mladic, but said their movements were being closely monitored and the information was classified.
"I want to warn anybody who in any way - either verbal, written, direct or indirect, by electronic messages or through carrier pigeons - communicates with fugitives from The Hague," he said.
His comments came just ahead of a debate in the Serbian parliament about co-operation with the UN war crimes tribunal in The Hague.
Mr Karadzic and Mr Mladic have been on the run since 1995. It is believed they are being sheltered somewhere in Serbia and Montenegro.
The BBC's Nick Hawton in Sarajevo says local media reports have suggested for some time that Mr Mladic has been in negotiations for his handover to the tribunal.
He adds that such a rare statement by Serbia's war crimes prosecutor is likely to heighten speculation that a breakthrough may be close.
Earlier this month, the international praised the arrest of leading Croatian war crimes suspect Gen Ante Gotovina in Spain. The European Union said it cleared a major obstacle to Croatia joining the EU.