A European Union deadline for Serbia to hand over war crimes suspect Ratko Mladic has expired with the former general still at large.
Gen Mladic has been indicted over the 1995 Srebrenica massacre
The EU said it would stop talks on closer ties with Serbia-Montenegro if Gen Mladic was not arrested by Sunday.
The former Bosnian Serb military chief is charged with genocide over the 1995 Srebrenica massacre and other crimes.
Along with his civilian counterpart, Radovan Karadzic, he is the most wanted war crimes suspect in Europe.
He is thought to be hiding somewhere in Serbia.
The BBC's Nick Hawton says this would not be the first time a deadline had passed without the arrest of Gen Mladic.
But he says this time there could be serious consequences for the Serbian government, which has made membership of the EU a key priority.
Serbia-Montenegro opened talks on a Stability and Association Agreement (SAA) with the EU - the first step on the path to membership - in October 2005.
Last month, the EU decided to carry on with the talks after assurances by UN Chief War Crimes Prosecutor Carla del Ponte and Serbian Prime Minister Vojislav Kostunica that Gen Mladic would be arrested.
But after meeting with Serbian Foreign Minister Vuk Draskovic on Friday, Mr Rehn told reporters that the EU had "no other option than to call off the next round of negotiations" planned for 11 May if Gen Mladic was not arrested within days.
Ms del Ponte is expected to give her assessment of Serbia's co-operation later this week. If her report is also negative then the EU may feel obliged to suspend future talks with Serbia, our correspondent says.