BELGRADE, May 17 (Reuters) - Top war crimes fugitives Ratko Mladic and Radovan Karadzic are hiding in Belgrade and Serbian authorities must find them, a spokeswoman for the U.N. war crimes prosecutor was quoted as saying on Thursday.

Belgrade's failure to arrest Mladic has been a major stumbling bloc in Serbia's progress to the European Union.

The EU froze talks on an association agreement last May, and says the new Serbian government, confirmed on Tuesday, must show concrete progress for talks to restart. But Mladic must be behind bars for a deal to be signed, the EU insists.

"Former Bosnian Serb leaders Radovan Karadzic and Ratko Mladic are in Belgrade," Olga Kavran, spokeswoman for prosecutor Carla del Ponte at the U.N. war crimes tribunal in The Hague, told the Serbian daily Danas.

Both men are indicted on two counts of genocide during the 1992-95 Bosnia war. Former Bosnian Serb political leader Karadzic has long been speculated to be in Russia, Bosnia or his native Montenegro.

Kavran said the tribunal had also passed on information to Serbian authorities regarding Mladic. Police raided a military boarding house in Belgrade on Tuesday night on a Hague tip-off, but did not find the former Bosnian Serb general.

Six ethnic Serbs are still on the run from the tribunal, having been indicted for crimes during the Croatia, Bosnia and Kosovo wars that followed the dissolution of Yugoslavia.

Kavran said Zdravko Tolimir, Goran Hadzic and Stojan Zupljanin were in Serbia, and Belgrade could use its influence to get Russia to arrest former general Vlastimir Djordjevic.

She said Serbian authorities had not cooperated with the tribunal for more than a year. In comments on Monday, the spokeswoman said the court would "patiently wait" for the new government to get in touch.

Rasim Ljajic, who heads the Serbian council for Hague cooperation, said Mladic's whereabouts were a mystery.

"No one can say whether Mladic is in Serbia or not in Serbia," he told Belgrade radio B92.

"None of the Hague indictees mentioned will be spared," he said. "Until we fulfil this obligation to The Hague we will have enormous domestic and foreign political problems."

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