At least 29 people were killed in Mexico when a passenger bus careened off a mountain road and plummeted into a ravine Wednesday in the southern state of Oaxaca, the state prosecutor said.

"The figures we have 12 hours after the rescue work began are 29 people dead and 19 injured," Bernardo Rodriguez Alamilla said by telephone.

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.

Fifteen women died, two of them while receiving hospital care, as well as 13 men and a child, the pr osecutor said.

The first official report set the number of fatalities at 27 and the number of wounded at 17.

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

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 meters (80 feet) deep," a state official, Jesus Romero, told a press conference.

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

Injured passengers were transferred to area hospitals, 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.

Deadly crashes

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

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

In 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.