The European Union has told Serbia and Montenegro that talks on closer ties will be put on hold unless it hands over war crimes suspect Ratko Mladic.
Gen Mladic has been indicted over the 1995 Srebrenica massacre
European commissioner Ollie Rehn told the BBC talks planned for early April would be called off until there was full co-operation from Serbia.
Mr Mladic is wanted in connection with the massacre of nearly 8,000 Muslims at Srebrenica and the siege of Sarajevo.
Serbia's defence minister has said he will resign if he fails to deliver.
Zoran Stankovic told the BBC that in recent months those supporting Mr Mladic had been identified and he hoped that he would soon be located and arrested.
He gave no timescale for the arrest, but said that if it did not happen well before the end of the year he would step down.
The BBC's Matt Prodger in Belgrade says that when Mr Stankovic was appointed in October, it was said that if anyone could secure the surrender of Ratko Mladic, it was him.
Serbia's Deputy Prime Minister Miroljub Labus says security forces have been told to arrest the general and he hoped this would happen by the end of March.
Mr Rehn said he believed the Belgrade government had the political will to bring Mr Mladic in.
He added that the army and security services had to obey the orders of the prime minister "to the letter".
Mr Rehn said it was necessary for a country wishing to join the EU to prove its army was under full democratic control.
"Serbia now has time roughly until the end of March, and I hope that full co-operation can be achieved by then, so as to avoid disruption and putting the negotiations on hold," he said.
UK Foreign Secretary Jack Straw told reporters that if Serbia failed to co-operate, it ultimately risked an end to the talks with the EU that began only last October.