ZAGREB, Croatia — At least two people were killed and several were injured when a bus overturned on a highway in Croatia on Sunday, police said.

The bus with Serbian license plates skidded off the road and overturned at 7:25 a.m. while traveling in the direction of the Croatian capital, Zagreb, police said. The reason for the accident was not immediately clear.

The A3 highway in Croatia leads from the Serbian border, by Zagreb and toward Slovenia.

Emergency services said that 33 passengers and two drivers were on the bus when it crashed. Around 20 people have been injured, some seriously, Dario Kezeric, Zagreb county firefighters’ commander, told the state HRT broadcaster.

Photos from the scene carried by local media showed a broken road fence and the bus lying on the side by the road. A helicopter was also called to the scene, the HRT report said.