The oil spill in the Black Sea that has hit Russian and Crimean coasts since mid-December is particularly hard to clean up because it involves a "heavy" fuel that is less likely to float.

The Black Sea is home to different cetacean species — some endangered — and it was the site of an oil spill in the middle of December that killed 31 of them.

Russia declared a regional state of emergency on Saturday in Crimea, which it seized from Ukraine in 2014, as workers cleared tons of contaminated sand and earth on either side of the Kerch Strait following an oil spill in the Black Sea last month.

An oil spill crisis has led to a regional state of emergency in Crimea. The situation arose from leaked oil from ageing tankers following a storm, affecting coastal areas and wildlife. Over 86,000 tons of contaminated sand and soil have been cleared,

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.

Oil from two ageing and damaged Russian tankers was detected Friday off the coast of Sevastopol, the largest city in Moscow-annexed Crimea, a local official said.