War crimes prosecutors have demanded the Bosnian Serbs hand over more than 20 suspects believed to be hiding in their territory.
"What we want is not a general commitment, what we want is active action," said Mr JeanJacques Joris, adviser to the court's chief prosecutor, Ms Carla del Ponte.
The mountains and forests of the Bosnian Serb Republic, the Serb part of Bosnia, are now the last refuge for most war crimes suspects. And with the arrest of former Yugoslav president Mr Slobodan Milosevic, war crimes prosecutors want closure.
Most wanted are the former Bosnian Serb president, Mr Radovan Karadzic, and his commander in chief, Gen Ratko Mladic, jointly accused of the worst atrocity in Europe since the second World War - the massacre of 7,000 Muslims in Srebrenica.
On Wednesday the republic's Prime Minister, Mr Mladen Ivanic, promised to back a new law which would increase co-operation. Yesterday Ms del Ponte told him to do more.
"That is not a strong enough statement," said Mr Joris. "The Serb Republic has so far put everything in the way against such arrests."
Under their constitution, the Bosnian Serbs have no discretionary powers to give or limit co-operation. Instead, they are bound to hand over all suspects unconditionally.
Mr Ivanic has picked a bad time to haggle. The UN court is in jubilant mood after the capture of Mr Milosevic. It knows the international community, in particular the US, is now backing its efforts after years of wavering.
When Mr Milosevic was in power, many Bosnian Serbs fled to Yugoslavia, notably Gen Mladic who occasionally gave interviews to journalists by shouting comments from the Belgrade villa where he lived. Now suspects have fled back to Bosnia.
The favourite place is eastern Bosnia. It has the highest mountains, deepest ravines and thickest forests. Also this sector is patrolled by the French army who have in the past been reluctant to arrest war crimes suspects. For the past four years NATO has played a cat-andmouse game with Serb suspects across Bosnia. More than 22 have been arrested by commando units, but almost all have fallen in the US and British-controlled sectors.
Mr Karadzic remains the key suspect: millions of pounds have been spent in searches for him, with the US at one stage sending elite commandos and helicopters with night-search equipment to try and track him down.
Yesterday his wife, Ljiljana, scotched reports earlier this week that Mr Karadzic was about to give himself up.
Mr Karadzic has one trump card - the sympathy of many ordinary Bosnian Serbs.
The wanted posters bearing his features on the trees and walls of the Serb Republic are outnumbered by a locally printed poster demanding "leave him alone" in English.
Reuters adds:
The Yugoslav President, Mr Vojislav Kostunica, railed against The Hague tribunal and Ms del Ponte in remarks published yesterday, calling the process selective justice and the prosecutor biased.
"At The Hague there is very little that could characterise a tribunal," Mr Kostunica said in Italy's Corriere della Sera and Serbian weekly Vreme.