"It crashed into the concrete road barrier before plunging into the ravine. Some people are still trapped inside the bus," Gumara said, adding no other vehicle was involved.

Search and rescue teams have been deployed to look for victims, Gumara said, adding injured survivors were taken to hospital.

The accident happened in a remote and steep area near the town of Pagar Alam, hampering the efforts to rescue the victims.

The regional bus left Bengkulu province for Pagar Alam with only 27 passengers but some survivors told police there were around 50 people on board when the accident happened.

"Some passengers were probably added along the way so there's a possibility some more people are still missing in the river," Gumara said, adding the cause was being investigated.

Traffic accidents are common in the Southeast Asian archipelago, where vehicles are often old and poorly maintained and road rules regularly flouted.

In September, at least 21 people died when a bus plunged into a ravine in West Java's Sukabumi region.

Several months earlier, 12 people were killed and dozens more injured when a passenger tried to wrest control of a bus steering wheel following an argument with the driver on the same toll road in West Java as Thursday's accident. The bus smashed into two cars, causing a truck to roll. —AFP