A passenger bus traveling to Montenegro has overturned on a poorly maintained road in Bosnia, killing three people and injuring a further 13.

The bus flipped over and fell into a ravine near the eastern town of Foca close to midnight Sunday, police said.

The dead are believed to have been thrown out of the bus and their bodies have still not been pulled out from under the wreckage.

Police are investigating the cause of the accident on the narrow, winding road connecting Bosnia with Montenegro that is considered one of the most dangerous routes in the Balkans.