A Siberian dam where 75 people were killed in a disaster last year is now threatened by a huge "iceberg" that has formed due to its stalled turbines and winter conditions, a Russian daily said Monday.

The ice mass, which weighs 25,000 tonnes and is 22 metres (72 feet) thick in some points, could lead to a new catastrophe at the Sayano-Shushenskaya hydroelectric power plant, two scientists told the Gazeta daily.

However the company which operates the plant, state-owned RusHydro, denied there was any risk to the dam straddling the powerful Yenisei River in the Khakassia region of southern Siberia.

Gazeta, which ran a photograph of the ice mass along the bottom of the dam, said it had formed because RusHydro was allowing water to flow out of the reservoir via a spillway which had never before been used in winter.

The August 17, 2009 accident damaged the turbines through which the water normally passes, forcing the dam operator to use the spillway to ease pressure on the gigantic dam.

A large cloud of mist thrown up from the spillway has been freezing and turning into ice flakes that fall on the roofs of various structures at the bottom of the dam, building up into a thick mass, Gazeta said.

"The situation is getting close to critical," Sergei Pashchenko, a researcher in the physics and mathematics department of Novosibirisk State University, told the newspaper.

The ice could cause parts of the power plant to collapse, said Pashchenko and another scientist quoted by Gazeta, Alexander Prokopchuk, a researcher at the nuclear physics institute of Moscow State University.

"The critical point will be when the ice reaches a height of 40-50 metres, or twice the current size. If the freezing temperatures continue, the iceberg could reach this size in two or three weeks," Prokopchuk said.

RusHydro said it was monitoring the situation and dismissed the possibility of another disaster at the dam.

"The situation is definitely not routine. Theoretically ice can be dangerous for any hydroelectric power plant. But in this case there is no risk of catastrophe," RusHydro managing director Rasim Khaziakhmetov told Gazeta.

A Russian government investigation concluded that the August disaster was caused by a technical fault in one of the plant's turbines and blamed senior officials, including some at RusHydro, for failing to prevent the disaster.