The EU has warned Serbia that moves towards eventual EU membership may be halted if Belgrade fails to hand over top war crimes suspect Ratko Mladic.
The former Bosnian Serb wartime general has been on the run since 1995. He has been charged with genocide and other crimes related to the Bosnian war.
EU Enlargement Commissioner Olli Rehn said suspending negotiations was an option if Belgrade did not co-operate.
The UN's chief war crimes prosecutor said Gen Mladic was hiding in Serbia.
Carla del Ponte accused elements in the Serbian army of sheltering him.
"Mladic is in Serbia, and as you know, Mladic is protected with power of the army," she said after a meeting with Mr Rehn.
She had urged Belgrade to hand over Gen Mladic to the war crimes tribunal in The Hague in 2005.
The former Bosnian Serb political leader, Radovan Karadzic, also tops the list of suspects most wanted by the tribunal.
Mr Rehn warned it would be extremely difficult for the EU to conclude an association agreement with Belgrade unless Gen Mladic and other wanted war crimes suspects were handed over.
An association agreement would be the first step on the road to EU membership for Serbia.
"I hope Belgrade takes this message very seriously and starts acting accordingly," Mr Rehn said.
"The suspension of negotiations is certainly one alternative... Serbia has to choose now between the nationalist past and a European future. I hope they choose the European future."
Ms del Ponte said she wanted to put Gen Mladic in the dock in July, along with the other suspects indicted for the killing of 8,000 Muslim men and boys in the Bosnian town of Srebrenica - the worst atrocity in Europe since World War II.
The Serbia-Montenegro Defence Minister, Zoran Stankovic, insisted that Belgrade was still looking for Gen Mladic, the Associated Press reported.
He said he had recently met Gen Mladic's wife and son.
"The operation to catch Mladic is under way... All available army personnel are engaged in this," he said.
Story from BBC NEWS: http://news.bbc.co.uk/go/pr/fr/-/2/hi/europe/4629174.stm