Russian officials have warned it is unlikely any of the 64 workers trapped within Russia's largest hydro-electric power plant after a blast will survive.

Twelve people are already confirmed dead after the explosion at the Sayano-Shushenskaya power plant in Siberia, the BBC reported.

Emergencies Minister Sergei Shoigu said groups of divers were searching for survivors in the flooded turbine hall.

The blast's cause is unknown but reports said investigators believed a transformer exploded during repairs. The plant's owner said the flooding had occurred due to a pressure surge in water pipes.

"Unfortunately we have no comforting news. We still do not know the fate of over 60 people," Shoigu told Russian state TV.

"The territory above water and under the water is being searched. We are awaiting a robot, which is able to work underwater," he said.

The plant's owners, RusHydro, said a detailed 3-D map of where its employees were at the time was being used during rescue efforts.

On Monday, two workers were rescued by divers from an area beneath the turbine hall.

But RusHydro's acting chairman said he did not hold out much hope of finding other workers alive.

"It is unlikely that survivors will be found where the flooding happened," Vasily Zubakin told Russian news agencies.

Wednesday has been designated as an official day of mourning in the remote mountain region of Khakassia where the plant - one of the country's largest - is located.

The accident has created a large oil slick that is now floating down the Yenisei river, which flows north through Siberia to the Arctic.

Officials said towns downstream of the plant were not thought to be at risk.