Interior Minister Bratislav Gasic described the latest shooting as a "terrorist act", Serbian news portal Telegraf reported, without providing further detail.
According to local media, after a late-night argument in a school yard near Mladenovac, 42 km (26 miles) south of Belgrade, the suspect returned with an assault rifle, opened fire and continued to shoot at people at random from a moving car.
State broadcaster RTS reported a policeman and his sister were among those killed.
Around 600 Serbian police, including elite Special Antiterrorist Unit (SAJ) and Gendarmerie launched a manhunt, dubbed Operation Whirlwind, for a 21-year-old suspect identified only as U.B, RTS reported.
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