Oleg Mitvol, deputy head of Russia's environmental regulator and a member of the United Nations' International Panel for Sustainable Resource Management, said the sinking of the ships in the Kerch Strait was a serious disaster and that it would take 10 to 15 years to clean up the strait and its coastline.

CNN: Death toll rises in oil disaster

Three cargo ships, each carrying more than 2, 000 tons of sulphur, also sank in the Kerch Strait, authorities said.

CNN: Death toll rises in oil disaster