Police, fire and medical teams were dispatched to the scene, which occurred at 5:40 am near Breznicki Hum in the direction of the capital Zagreb, police said.

“We have 43 injured, 12 of them have died,” said Maja Grba-Bujevic, director of the Croatian Institute of Emergency Medicine. “All the victims are Polish citizens – at the moment we can confirm this,” a spokesman for the Polish Foreign Ministry said. "The bus has Warsaw numbers."

The passengers were pilgrims bound for Medjugorje, a Roman Catholic shrine in southern Bosnia, Croatian Interior Minister Davor Božinović told reporters.

The cause of the accident is being investigated.