Russia has warned of severe environmental damage from a huge oil spill in the Black Sea caused when two tankers were hit by a storm near Crimea, which has declared a state of emergency.

One tanker sank and another ran aground on 15 December in the Kerch strait between Russia and the illegally annexed Crimean peninsula.

Thousands of volunteers have been mobilised for clean-up operations that have been criticised as insufficient by some Russian scientists.

The tankers were carrying 9,200 tonnes of fuel oil, about 40% of which may have spilled into the sea, according to authorities.

“The situation is truly critical,” said the Kremlin spokesperson, Dmitry Peskov, as quoted by Russian press agencies.

“It is unfortunately impossible to calculate for the moment the extent of the environmental damage, but specialists are working regularly on it,” he said.

The Moscow-installed governor of Crimea, Sergei Aksionov, said on Telegram he had declared a state of emergency “because of the oil products spill in the Kerch strait”.

Russia’s transport ministry said on Saturday that “all polluted aquatic areas that have been identified have been cleaned” and “no recurring pollution has been detected”.

But the emergency situations minister, Alexander Kurenkov, was more cautious. He said: “The threat of a new fuel oil leak in the Black Sea from the tankers and spills on the coast persists.”

Vladimir Putin earlier this month called the oil spill an “ecological disaster”.