OAXACA, July 6 — At least 27 people were killed in Mexico when a passenger bus careened off a mountain road and plummeted into a ravine yesterday in the southern state of Oaxaca, police said.

Images from the scene showed the mangled wreckage of the vehicle lying on its side at the bottom of a steep cliff as rescuers worked nearby.

"According to a preliminary toll, 27 people died and 17 injured were transferred to different hospitals in the region for medical attention,” Oaxaca state prosecutor Bernardo Rodriguez Alamilla told AFP by telephone.

An investigation has been launched into the cause of the accident, with early indications suggesting mechanical failure, he added.

At least six of the injured were unconscious and in serious condition when they were rushed to hospital, according to the civil protection agency.

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

"The driver of the vehicle presumably lost control... and unfortunately it fell into a ravine more than 25 metres deep,” a state official, Jesus Romero, said at a press conference.

The company operating the bus provides a daily service from Mexico City, he said.

Injured passengers were transferred to hospitals in the area, while emergency services retrieved the bodies of those killed, Romero added.

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.

Deadly road accidents are common in Mexico, usually due to high speeds, poor vehicle conditions or driver fatigue.

Many people rely on buses, sometimes operated by small transportation companies serving remote communities with aging vehicles.

Crashes involving freight trucks have also increased on the country’s highways.

On Wednesday, a fiery smash involving cargo vehicles left eight people injured on a highway in the central state of Queretaro, authorities said.

In May, at least 18 Mexican tourists died when their bus plunged into a ravine in the western state of Nayarit.

The same month, 13 people were killed when a passenger van and a semi-truck collided on a highway in the northeastern state of Tamaulipas.

Road safety campaigners have called for stricter regulations, such as a ban on trucks pulling two cargo trailers. — AFP