Anton Nikiforov, Carlo del Ponte's acting spokesman, said on Thursday the promise to deliver the former Bosnian Serb military commander indicted for genocide came from Vojislav Kostunica, the Serbian prime minister.
"We received clear assurances from Kostunica for the delivery of Ratko Mladic by the end of April," Nikiforov told reporters in The Hague.
Belgrade had also made the same pledge to Oli Rehn, the EU enlargement commissioner, he said.
The arrest and handover of Mladic, indicted twice for genocide in the 1992-95 Bosnia war, is a precondition for closer ties between Serbia and the European Union.
The promise was sufficient for the EU to allow association talks with Belgrade to go ahead this week, but EU officials told Serbia on Wednesday that further progress on closer ties with the bloc would depend on the arrest of Mladic.
Belgrade has said it is doing all it can to deliver Mladic, and there is constant speculation in Serbian media that he is considering surrender or is ready to die to avoid arrest.
"You can see there were some things happening in Belgrade but we have no concrete information as to what is being done," Nikiforov said, without disclosing further details.
Del Ponte has said the death of Slobodan Milosevic, the former Yugoslav president, last month before a verdict in his war crimes trial has made arresting Mladic all the more urgent.
Del Ponte says Mladic is hiding in Serbia with help from supporters in the army and intelligence service.
Mladic commanded Bosnian Serb forces and is indicted for genocide along with his political boss, Radovan Karadzic, for the Srebrenica massacre of 8,000 Muslims in 1995 and the long siege of Sarajevo in which more than 10,000 civilians died.