The victims were explosives experts working on the Chinese-financed Sopladora plant, a project leader told AFP today.
"These people were working on the expansion of a loading pit at the time of the explosion yesterday, whose cause is still being investigated," Tito Torres, manager of state-run Hidropaute, the company in charge of construction, said today.
The project, which began in 2011, is part of a network of eight hydroelectric plants that are being constructed.
Oil-producing Ecuador hopes that the project will enable it to stop importing electric power and become an exporter of clean energy.
Sopladora, with a capacity of 487 megawatts, is expected to be operational in 2015.