MOSCOW: A sudden flood at Russia's largest hydroelectric plant killed at least 12 people and left dozens missing after a surge in water pressure ruptured pipes, officials said Tuesday.

The accident Monday caused serious power disruption in Siberia, cutting off electricity to smelters of major Russian metals manufacturers UC Rusal and Evraz Group.

It happened when a sudden increase of water pressure in pipes feeding a power unit caused the rupture at the Sayano-Shushenskaya plant in the Khakassia region.

According to the latest toll given Tuesday by Russia's Minister of Emergency Situations Sergei Shoigu, who was at the scene, 12 people were dead and 64 missing, Interfax news agency said.

The plant's chief engineer, Andrei Mitrofanov, told Itar-Tass that "around 300 people" would have been on the ground at the time of the accident.

The power station is one of the most powerful in the world with a capacity of 6.4 million kilowatts an hour.

The components were undergoing repairs when the accident occurred, causing a large portion of the power unit to break off and puncture the ceiling and wall, allowing water to pour in and flood the chamber.

In a statement, the Kremlin said the accident was due to an unspecified "hydraulic impact" at the plant which forced the shutdown of all 10 of the station's power units.

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

Prime Minister Vladimir Putin arrived Monday night at the headquarters of the national crisis management centre.

Medvedev sent a message of condolences to employees of the plant.

"We are going to look carefully into the causes of this catastrophe and provide all necessary assistance to the injured," Medvedev said, according to the Kremlin press office.

The accident sparked panic among nearby residents who feared the massive dam at the facility might collapse, but Shoigu said there was no threat to people downstream.

"Towns and villages located downstream are not in danger," Shoigu told reporters in Moscow.

Andrei Klyuyev, an emergency situations ministry official at the site of the accident, 4,000 kilometres (2,500 miles) east of Moscow, said there were still dozens of people unaccounted for.

Klyuyev said rescue divers had pulled out one person from a room underneath the plant's turbine hall where there was apparently a cave-in and flooding but said many more could still be trapped.

Russian television broadcast pictures showing pieces of concrete, cables and twisted metal pylons beneath the 245-metre high dam wall and the main turbine hall flooded with water and debris.

The accident disrupted power supply to key smelters in the region including those of UC Rusal, Russia's largest aluminium producer, and other enterprises.

A Moscow-based spokeswoman for Rusal, which is controlled by billionaire businessman Oleg Deripaska, said however the operation of the smelters had not been seriously affected as power had been diverted from alternative sources.

The company said in a statement later however that Deripaska had discussed with Shmatko the possibility of reducing output from UC Rusal to free up energy resources to ensure "stable functioning of the region".

"The situation has become extremely difficult following the accident," the governor of the Tomsk region, Viktor Kress, told RIA Novosti. "It has left us down around 41 megawatts of power."

Russia's financial regulators ordered the suspension of trading on both Moscow stock exchanges in shares of state-run RusHydro, the corporation that owns the affected hydroelectric station.

Konstantin Reily, a utilities analyst at Finam, estimated that it might cost up to three billion dollars to replace the three damaged power units.

"This is an extraordinary event. This is the first accident of such a scale at the Sayano-Shushenskaya hydroelectric plant," he added.

Rebuilding the plant would take "four years or more", Vasily Zubakin, the acting chairman of RusHydro, told RIA Novosti.

The mayor of the nearby town of Abakan, speaking to Echo of Moscow radio, said queues had begun forming outside bakeries and gas stations.

The natural resources ministry said it was concerned by the environmental impact of the accident, saying an oil slick of more than 25 kilometres (15 miles) had spread along the Yenisei River.

"According to preliminary data, transformer fluid has leaked from one of the hydroelectric station's damaged units," the ministry said in a statement.