On December 15, two Russian oil tankers, the Volgoneft-212 and the Volgoneft-239 were hit by a storm in the Kerch Strait, with one sinking and the other running aground.

The strait separates southern Russia from the Ukrainian peninsula of Crimea, which it annexed in 2014.

The ships were carrying 9,200 tonnes of fuel oil, around 40 percent of which may have spilled into the sea, according to Russian authorities.

President Vladimir Putin last week called it an "ecological disaster".

Thousands of volunteers were mobilised to remove oil-sogged sand from nearby beaches.

But scientists say the volunteers don't have needed equipment.

"There are no bulldozers there, no trucks. Practically no heavy machinery," said Viktor Danilov-Danilyan at a news conference.

Danilov-Danilyan is the scientific head of the Water Problems Institute of the Russian Academy of Sciences and served as Russia's environment minister in the 1990s.

The volunteers have only "shovels and useless plastic bags that rip apart", he said.

"While the bags wait to finally be collected storms arrive and they end up back in the sea. It's unthinkable!"

Public criticism of the authorities is rare in Russia.

Up to 200,000 tonnes of sand may have been contaminated with oil, Russia's minister of natural resources said on Monday.

Nearly 30,000 tonnes have already been collected, said Krasnodar region Governor Veniamin Kondratyev on Wednesday.

Sergei Ostakh, a professor at the Russian Academy of Natural Sciences, said the oil could soon reach shores in Crimea.

"No one should have illusions it will stay clean," he said, calling for quick action.

The oil spills may have killed 21 dolphins, the Delfa dolphin rescue centre said, although additional tests were needed to confirm the cause of death.