At least 17 people were killed and 31 were injured when a bus plunged some 300 feet off a roadside in a mountainous region of southern Mexico near the Guatemalan border, authorities said on Sunday.

The accident occurred near the town of Motozintla when the bus ferrying people home from a spiritual retreat on the Pacific coast came off the road and fell down a steep slope, emergency services in the state of Chiapas said in a statement.

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The passengers came from the nearby municipality of La Trinitaria and at least 17 died and another 31 were injured, said a spokesman for emergency services, Daniel Cuate.

It was not immediately clear what caused the accident.

President Enrique Pena Nieto expressed his condolences to the families of the bereaved on Twitter.

Photographs from the scene showed the bus had been reduced to a mangled wreck in a wooded gully.