Earlier reports said at least six people were killed and seven injured in the accident.

Rescue officials said the toll could rise as people may still be trapped in the flooded rooms.

The accident may have been caused by a hydraulic surge, or a sharp increase in pressure, said Kudryavtsev.

Property worth hundreds of millions of rubles may have been damaged in the accident, said Yevgeny Druzyaka, an official of RusHydro, Russia's largest power generator.

He said the repair work has already started at the plant.