Russian investigators have determined that an oil spill from two tankers in the Black Sea last month was smaller than initially thought, the Transport Ministry said on Thursday.

The oil leaked from two ageing tankers that were hit by a storm on Dec. 15. One sank and the other ran aground.

More than 10,000 people have been working to shovel up viscous, foul-smelling fuel oil from sandy beaches in and around Anapa, a popular summer resort. Environmental groups have reported deaths of dolphins, porpoises and sea birds.

The ministry said experts had established that approximately 2,400 metric tons of oil products had spilled into the sea.

"This is significantly less than the initial estimate, which was based on the account of one of the tanker captains," it said.

When the disaster struck, state media reported that the stricken tankers, both more than 50 years old, were carrying some 9,200 metric tons (62,000 barrels) of oil products in total.

The ministry said the spill involved heavy M100-grade fuel oil that solidifies at a temperature of 25 degrees Celsius and, unlike other oil products, does not float to the surface but sinks to the bottom or remains suspended in the water column.

"There are no proven technologies in the world to remove it from the water column. Therefore, the main method is collection from the shoreline," the ministry said.