Russia is ramping up its efforts in the Ukraine war by doubling the number of its pens holding trained dolphins near the Black Sea port of Sevastopol, the UK defense intelligence agency said on Friday. The British bureau’s satellites captured images of expanding enclosures it said contain bottle-nosed dolphins.

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“The implication is that dolphin patrols have increased in frequency and/or cover more area,” according to Naval News. “The marine mammals have an inherent advantage against even the most athletic combat swimmer; no one can out-swim a dolphin.”

The Russian navy uses Beluga whales and seals in Arctic waters. The UK Ministry of Defense said on Friday: “Russia has trained animals for a range of missions, but the ones housed in Sevastopol harbor are highly likely intended to counter enemy divers.”

The Ministry of Defense said in its report that the additional pens include “at least four layers of nets and booms across the harbor entrance.” It added: “In recent weeks, these defenses have highly likely also been augmented by an increased number of trained marine mammals.”

The report of additional dolphin pens comes after Russia accused Ukraine of launching drone attacks on its Sevastopol base earlier this month. Crimea’s governor, Mikhail Razvozhaev, claimed “more than ten drones” attacked Crimea and Sevastopol, and added that their “air defense forces and electronic warfare units prevented another attack on Sevastopol,” he said in a statement shared on Telegram.

Sevastopol is a city and major port in Crimea, which Russia illegally annexed in 2014, and is still recognized internationally as being part of Ukraine. Russia has continued to bolster its military presence in Crimea’s Sevastopol port since the summer of 2022, according to Naval News.

The intelligence agency has followed the Kremlin’s advances in Ukraine since Russia invaded the country in February 2022. The dolphins are strategically located at a port that “could prevent Ukrainian special operations forces from infiltrating the harbor underwater to sabotage warships,” the United States Naval Institute said in a statement last year. However, it is unclear whether this is part of Ukraine’s plan.

Dolphins have been used by both the U.S. and Russia’s militaries in past years, particularly during the fall of the Soviet Union in 1991. Dolphins were used during the Cold War to detect submarines, and mines, and locate objects deemed suspicious near its harbors and ships. The U.S. Navy discovered by studying Notty, a female Pacific white-sided dolphin, that dolphins could become an asset on the battlefield. “It was soon after discovered that they had excellent biological sonar, so they definitely did a lot of research on that as well,” Ed Budzyna, a spokesman for SPAWAR Systems Center Pacific, told Business Insider in 2015.

In the years since the Cold War, Russia has been reported to continue mobilizing dolphins and whales, one of which appeared to have escaped its pen in 2019. The Beluga whale was spotted in Norway wearing a harness carrying the words “Equipment St. Petersburg,” and experts believed it was used for Russian surveillance. Marine Biologist Jorgen Ree Wiig told CNN at the time that “the whale seemed playful, but our instincts said that it was also asking for help to get out of the harness.” Marine biologists from the Norwegian Directorate of Fisheries removed the harness and have continued to track the whale, which resurfaced off the coast of Sweden in May.