Olli Rehn, the EU enlargement commissioner, on Monday warned Belgrade that talks scheduled for April could be postponed if Mladic was not handed over for trial at the UN war crimes tribunal.
Rehn, who is leading the EU talks with Belgrade on a pre-membership aid-and-trade deal, had set an informal end-of-February deadline for Belgrade to fully co-operate.
"It means that there is some more time until the end of March" for them to co-operate with the tribunal, Rehn spokeswoman Krisztina Nagy said.
Rehn told the EU foreign ministers that if no progress is made before the next round of negotiations, planned for 5 April, those talks could be frozen.
EU foreign ministers called on both Serbia-Montenegro and Bosnia to "take resolute measures ... to bring to justice" Mladic and other top war crimes suspect, Radovan Karadzic, the Bosnian Serb leader.
Jack Straw, the British foreign secretary, said the EU had to get tough with Belgrade, just as it did with Croatia last year, to hand over indicted war criminals.
Straw said: "The message has to be unless these countries like Serbia co-operate with the criminal tribunal in The Hague, and hand over indicted war criminals, they cannot expect full cooperation from the European Union in return."
Mladic is said to be enjoying the
protection of Serbian hardliners
Rehn and Ursula Plassnik, the Foreign Minister of Austria (which holds the EU presidency), were to meet Vuk Draskovic, Serbia-Montenegro's Foreign Minister, later on Monday to discuss his country's co-operation with the UN tribunal.
Last week the media reported that authorities were closing in on Mladic, but he remains at large.
Mladic and Karadzic were indicted in 1995 on charges of orchestrating the massacre of about 8000 Muslims in the UN enclave of Srebrenica - Europe's worst carnage since the second world war.
Mladic is believed to be hiding in Serbia under protection of hardliners in the Serb military and police - loyalists of Slobodan Milosevic, the Yugoslav president who was toppled in 2000 by a reformist coalition.
Karadzic reportedly has been hiding and moving between Bosnia, Serbia and even his native Montenegro.