The spill began on December 15, when two ageing Russian tankers were caught in a storm off the Kerch Strait linking Crimea to southern Russia.

One sank and the other ran aground, pouring around 2,400 tonnes of a heavy fuel oil called mazut into the surrounding waters, authorities estimate.

Russia's Delfa centre, which rescues and rehabilitates dolphins, said it had recorded 61 dead cetaceans since the incident, 32 of whom "most likely" died due to the spill.

Cetaceans are a type of aquatic mammal that includes whales, dolphins and porpoises.

"Judging by the condition of the bodies, most likely the bulk of these cetaceans died in the first 10 days after the disaster," it said on Telegram.

It said most of those killed were "Azov" dolphins -- a type of harbour porpoise that look similar to dolphins but are more closely related to belugas and narwhals.

Russia's emergency ministry said Sunday it was working to clean up after the incident, but that "strong winds and waves" had thrown oil onto some beaches.

"More than 68 kilometres (42 miles) of coastline have been cleaned," it said.

In a further statement Sunday, the ministry said two new oil slicks had been discovered.

One was off the seaside resort of Anapa and the other in the bay of Kapsel, it said in a statement cited by the Russian news agency TASS.

The news agency said the second slick was two kilometres long.

Hundreds of volunteers have been deployed to scoop up contaminated soil from beaches in Crimea and along Russia's southern coast.

The type of fuel oil involved in the incident is particularly hard to clean because it is dense and heavy and does not float on the surface, Russian authorities say.