At least 2,500 endangered seals have been found dead along Russia's Caspian Sea coast, the local environment ministry said. 

Investigators are still working to explain why the seals washed up on beaches in Dagestan, southern Russia.

Authorities are leaning towards suffocation from gas released from the seafloor as the "main" cause of death, said Svetlana Radionova, the head of the Russian agency for natural resources.

She said she expected the first results of the investigation -- which should determine if water pollution had an impact -- towards the end of the week.

In an interview on Russian television Rossiya-24, Radionova recalled a similar disaster that saw 2,000 dead seals wash up on the coast of Dagestan and Azerbaijan in 2020. 

The Caspian Sea, the world's largest inland body of water, is bordered by five countries: Russia, Kazakhstan, Azerbaijan, Iran and Turkmenistan.

The seal population of the Caspian Sea has for decades suffered from over-hunting and the effects of industrial pollution.

Experts say there are now about 68,000 Caspian seals, down from more than one million in the early 20th century.

Pollution from the extraction of oil and gas there, along with declining water levels due to climate change, pose a threat to many species and put the future of the sea itself at risk.