BELGRADE, May 4 (Reuters) - Serbia could arrest four war crimes fugitives still on the run from the Hague tribunal, including top suspect Ratko Mladic who is likely in the country, the U.N. chief prosecutor was quoted on Sunday as saying.

The arrests are key to Serbia's progress towards European Union membership. A pact on closer ties, signed between Belgrade and the bloc last week, is due to go into effect only when all, and especially Mladic, are brought to trial.

Hague chief prosecutor Serge Brammertz, the arbiter of whether Serbia is doing its best to capture the four men, told Serbian daily Vecernje Novosti "there is no reason to believe Mladic is not in Serbia."

"His support network is (in Serbia), his family is in Serbia, and that is where he was last seen," Brammertz said.

Along with his political boss Radovan Karadzic, Bosnian wartime commander Mladic is indicted on two counts of genocide, for the Srebrenica massacre of 8,000 Bosnian Muslims and for the 43-month siege of Sarajevo during the 1992-95 Bosnia war.

Commenting on the other fugitives -- Karadzic, Stojan Zupljanin and Goran Hadzic -- Brammertz told the paper the tribunal believes that "Serbia can get to all three".

"During meetings in Belgrade we received detailed information about a recent raid confirming Zupljanin was in Serbia," he said.

The fugitives' families and support networks were in Serbia, he added, so there was no reason to believe they were far away.

Brammertz, a Belgian, took over the Hague post in January and is due to make his first formal report on Serbia's cooperation to the U.N. Security Council in the coming weeks.

He replaced Switzerland's fiery Carla del Ponte, who for eight years put relentless pressure on Belgrade to deliver the fugitives. Many Serb nationalists consider Mladic and Karadzic heroes for their role in the Yugoslav wars. (Writing by Ellie Tzortzi; Editing by Dominic Evans)

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