Russian President Vladimir Putin said on Thursday that more needed to be done to clean up an oil spill in the Black Sea, saying efforts so far appeared to have been insufficient to deal with the ecological disaster.

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 Russian government made the declaration after a number of its tankers were sank in the body of water between itself, Ukraine, Turkey and Romania.

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.

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.

President Vladimir Putin on Thursday criticized Russian officials for their slow response to what he described as the country’s most serious environmental disaster in recent memory. “This is practically one of the most serious environmental challenges we’ve faced in recent years,