UN war crimes prosecutor Carla del Ponte has urged the EU not to resume talks on closer ties with Serbia until it hands over war crimes suspects.
Ms del Ponte wants to see more co-operation from Belgrade
Speaking after talks with EU foreign policy chief Javier Solana, she said Serbia knew the whereabouts of one of the main suspects, Gen Ratko Mladic.
Some EU members now favour a softer stance towards Belgrade, reports say.
This comes as a UN envoy is expected to recommend conditional independence for the Serbian province of Kosovo soon.
Ms del Ponte said it would send the wrong signal to Belgrade if talks resumed now.
The EU suspended talks with Serbia last year over Belgrade's failure to capture the Bosnian Serb general.
"That is my worry, that it could happen and jeopardise what we absolutely need now, because it's a crucial time now for us in co-operation with Belgrade," Ms del Ponte said.
"The old government, the actual government or the new government, must [understand] the conditionality that Mladic must be transferred to The Hague."
She said she was reassured by Mr Solana, whom she met in Brussels.
He is set to go to Belgrade next week for talks with the pro-Western parties hoping to form a new government after general election earlier this month.
But the EU is divided over Serbia, says the BBC's European Affairs correspondent Oana Lungescu.
While Italy, Spain and Serbia's central European neighbours are pushing for talks to resume, the UK, France and the Netherlands want firm guarantees that co-operation with the war crimes tribunal will yield results, our correspondent says.
Ms del Ponte is going to Rome and Madrid next week to warn that while Belgrade is doing nothing to catch the former Bosnian Serb military commander, resuming negotiations would send a bad signal; it would also mean a failure for international justice.
She confirmed that after eight years in the job, she planned to retire in September.
Gen Mladic is wanted by The Hague war crimes tribunal on genocide charges relating to the massacre of thousands of Muslim men and boys in Srebrenica in 1995 during the Bosnian war.