The bus, carrying 30 passengers, overturned on the road between the towns of Garumales and Mendez in eastern Morona-Santiago province yesterday, said Fernando Figueroa of the Integrated Security System Ecu-911.
Local officials had earlier said there were more than 40 passengers and that most were workers building a road connecting the two towns.
The bus "fell down to the banks of the Bomboiza River," Figueroa told state broadcaster ECTV.
An Ecu-911 representative in the province's capital Macas said all the injured people had been placed in medical centres, some of them in critical condition, in Cuenca.
In November, 15 people were killed and more than a dozen were injured after a bus plunged some 500 meters (1,640 feet) into a ravine as it travelled from the southeastern town of Manta to Quito.
And in May, 12 college students were killed and 15 injured when the bus they travelled in fell into another abyss of some 150 meters.