MOSCOW: Hopes dwindled Tuesday over the fate of 64 workers feared trapped in a flooded turbine hall at a Siberian hydroelectric power station after one of Russia's worst energy disasters of recent times.

Divers were searching the flooded wreckage at the Sayano-Shushenskaya plant after Monday's accident but officials said that with water at just 4 degrees Celsius (40 degrees Fahrenheit) the chances of finding more survivors were low.

The catastrophe -- already confirmed to have left 12 people dead -- is believed to have been triggered when a technical problem caused a massive surge of water to erupt into the turbine hall, engulfing 100 workers.

"Unfortunately we have no comforting news. We still do not know the fate of over 60 people," a grim-faced Russian emergencies situations minister Sergei Shoigu told Russian state television from the region.

"The territory above water and under the water is being searched. We are awaiting a robot, which is able to work underwater. Three groups of divers are now working under the water," he added.

Russian state television said 1,000 specialists were involved in the search operation around the turbine hall, where the water was still estimated to be 20 metres (65 feet) deep.

"It is unlikely that survivors will be found where the flooding happened. We are carrying out search operations," added Vasily Zubakin, acting chairman of the plant's operator RusHyrdro, according to Russian news agencies.

He confirmed that 64 people were still missing. No new information on their condition filtered out in the course of the afternoon.

Rescue divers had on Monday pulled out two people alive who were suffering from hypothermia but were otherwise unharmed.

Fatal accidents are frequent occurrences at often ageing energy facilities across the former Soviet Union. But the scale and manner of the potential loss of life makes this accident exceptional.

Officials from RusHydro initially said the accident had been caused by a sudden increase in water pressure but later added that a broken turbine could have been at fault.

"Instruments indicated that it was not a hydraulic impact but a broken turbine cover in the machine hall," said RusHydro board member Alexander Toloshinov, according to the Interfax news agency.

Vladimir Markin, a spokesman for the investigative committee of prosecutors, also said investigators "had not found any explosive substances at the scene of the disaster" and ruled out terror as an explanation.

But he said that a criminal inquiry had been opened for neglecting the rules of safety at work.

Local officials have said that Wednesday will be an official day of mourning in the local region of Khakassia, a remote area in the mountains of southern Siberia.

The power station -- whose massive dam across the Yenisei River is a stunning local landmark -- has now shut down completely amid questions about how the region will make up the energy shortfall.

The station which was built under the Soviet Union between 1968 and 1985 is the biggest in Russia and also one of the most powerful in the world with a capacity of 6.4 million kilowatts an hour.

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

Russia's financial regulators ordered the suspension of trading on both Moscow stock exchanges in shares of state-run RusHydro.

Rebuilding the plant will take four years at a cost of 10 billion rubles (222 million euros, 314 million dollars), Zubakin said. Three generators were ruined although the massive dam remains intact.

However, there has been concern about the environment, with Shoigu saying the accident has produced an oil slick along the river that now stretches up to 60 kilometres (37 miles) long.