The Russian Navy is increasing the number of trained dolphins it uses to protect its main military base in the Black Sea, according to intelligence reports.

The animals are guarding the entry to the port of Sevastopol, in Russian-occupied Crimea, and are likely intended to “counter enemy divers,” British military intelligence said Friday.

In recent weeks, “imagery shows a near doubling of floating mammal pens in the harbor which highly likely contain bottle-nosed dolphins,” the report says.

Trained animals have been used for decades by the military or intelligence agencies to carry out specific missions. A Beluga whale which has made several appearances off the Scandinavian coast in recent years is for instance believed to be a spy trained by the Russian army.

Russia’s Black Sea fleet has been targeted by several drone attacks since the start of the Kremlin’s full-scale invasion of Ukraine in February 2022.

In April, Russian ships stationed in Sevastopol were hit by an attack carried out with “three unmanned high-speed boats,” and a fuel depot in the same city was hit by a drone strike a few days later — prompting Moscow to announce a tightening of security measures at the Sevastopol naval base last month.

Ukraine has been accused by Russia of being responsible for these attacks, but has avoided taking responsibility for them.

Russia illegally annexed the Crimean peninsula in 2014.