A bus carrying Polish tourists overturned north of Belgrade on Friday, killing six people and injuring nearly 40, Serbian police said.

Police said 68 people were on the bus when the accident happened at about 6.30 a.m. (0430GMT) on the road between the Serbian capital, Belgrade, and the northern city of Novi Sad.

The Polish bus “for a so far unknown reason swerved off the road and overturned, falling on its right side,” a police statement said.

Two children, a woman and three men were killed, and nearly 40 people were admitted to the hospital in Novi Sad, seven of whom were seriously injured, police said.

Serbian state television reported the tourists were returning to Poland from a holiday in Bulgaria.

In Poland, the PAP news agency quoted the owner of the Moana bus company, Josef Rzepka, as saying the two drivers of the vehicle were unhurt.