Around 2,500 seals have been found dead on the Caspian Sea coast in southern Russia, officials have said.

Authorities in the Russian province of Dagestan said it remained unclear what had caused the animals’ deaths, but that it was likely they had died of natural causes.

Regional officials initially said on Saturday that 700 dead seals had been found on the coast, but the Dagestan division of the Russian Ministry of Natural Resources and Environment later raised the figure to about 2,500.

Zaur Gapizov, head of the Caspian Environmental Protection Centre, said in a statement that it was likely the seals had died a couple of weeks ago. He added that there were no signs they had been killed or caught in fishing nets.

Federal Fisheries Agency experts, together with prosecutors, inspected the coastline and collected data for laboratory research, which did not immediately spot any pollutants.

Several previous incidents of mass deaths of seals have been attributed to natural causes. Kazakhstan, which has a long Caspian coastline, has reported at least three such incidents this year.

The data about the number of seals in the Caspian varies widely.

The fisheries agency has said the overall number of Caspian seals is between 270,000 and 300,000, while Mr Gapizov’s centre puts the number at 70,000.