At least 25 people were killed in Mexico when a passenger bus plummeted into a ravine Wednesday in the southern state of Oaxaca, police said.

"The preliminary toll is 25 people dead and 17 seriously injured," a police officer said by telephone, asking not to be identified because he was not authorized to speak to the media.

The bus, operated by a local transport company, had left the capital Mexico City and was heading for the town of Santiago de Yosondua, authorities said.

The accident happened in Magdalena Penasco, a town located in a mountainous area home to remote communities, winding roads and steep ravines.

"We deeply regret the accident that occurred in Magdalena Penasco," Oaxaca state governor Salomon Jara wrote on social media, offering condolences to the families of the deceased.

"Our government personnel are already working on the rescue operation and to provide all the support to the injured people," he said.

Police images published on social media showed the top half of the bus almost completely destroyed, while rescuers searched through the debris.