At least 64 people are missing after an explosion at Russia's largest hydro-electric power station killed twelve workers.

An oil-filled transformer exploded at the Sayano-Shushenskaya plant in Siberia, bringing down the ceiling of the turbine hall, which then flooded, the BBC reported.

A diver has pulled one worker from an area beneath the turbine hall, but it is unclear how many may be trapped, officials said.

Ten workers were injured, but there was no damage to the power station's dam.

However, 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.

Russian President Dmitry Medvedev has ordered Emergencies Minister, Sergei Shoigu, and Energy Minister, Sergei Shmatko, to fly to the scene, and take personal control of the crisis.

RusHydro, the operator of the power station, said the damage would run into "billions of roubles" and would take several months to repair.

The Sayano-Shushenskaya power station is located in the Siberian region of Khakassia, some 3 000 km east of Moscow.

The dam above it is 245m high and stretches 1km across the Yenisei river.

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