The accident occurred when the driver left the bus parked on a steep incline putting rocks under the wheels to prevent the vehicle from moving. However, the driver's actions failed to stop the vehicle, which was overloaded with passengers, from rolling backwards and over the edge of a sharp drop.

"The tragedy was caused by the negligence of the driver, who neglected elementary safety rules," the source said.

Four women were killed in the accident, while some 10 people were injured, six of them seriously.

An investigation is underway.