The bus was traveling back to Slovakia from Greece when it slid off near the central Serbian town of Aleksinac. It was carrying 29 passengers and two drivers.
Serbia's Health Minister Zlatibor Loncar visited the injured in a hospital in the southern city of Nis. He said a 55-year-old woman remains in serious condition.
Serbian state television said a plane from Slovakia is due in Nis later Tuesday to pick up the survivors. It was not immediately clear what caused the bus to swerve.