Russia has significantly increased the number of pens containing trained dolphins near the strategic Black Sea port of Sevastopol, to help protect its naval forces, according to UK intelligence.
Russia, which has a history of using marine animals in its military, has doubled the number of marine mammal pens in and around the Sevastopol harbor, the UK Ministry of Defence said in an intelligence update on Friday.
The MOD described this as part of Russia's investments in "major enhancements to the security of the Black Sea Fleet's main base at Sevastopol" since summer 2022.
Russia invaded Ukraine in February 2022, and Sevastopol, a city and major port on the Black Sea, is in Crimea, a region of Ukraine that Russia annexed in 2014 and has controlled ever since. Russia's claim to Crimea has not been recognized internationally.
The MOD said Russian increases to the fleet's security include "at least four layers of nets and booms across the harbour entrance."
"In recent weeks, these defenses have highly likely also been augmented by an increased number of trained marine mammals," it said.
It added that the animals "are highly likely intended to counter enemy divers."
Placing dolphins at the mouth of the port could stop Ukrainian special operations forces getting into the harbor underwater, United States Naval Institute said earlier this year, though it noted that it is not clear if Ukraine has any plans to do this.
Satellite images captured by Maxar Technologies last April also showed militarized dolphin pens near Sevastopol.
The MOD's update came after a similar report from Naval News last week.
Russia has been training dolphins in the Black Sea since the Cold War, according to the United States Naval Institute.
The MOD also noted Russia's documented use of marine animals elsewhere, saying "Russia has trained animals for a range of missions." It cited Russia's navy using beluga whales and seals in the Arctic.
One recent case involved a beluga whale seen in Norway in 2019 that was wearing a harness that read "Equipment St. Petersburg." It was later seen off the coast of Sweden. Experts said it might have been involved in spying.
The US Navy has also militarized dolphins, using them to clear mines during the Iraq War.
Russia's Black Sea fleet includes attack submarines and vessels with long-range strike capabilities and air defense missiles.
Russia accused Ukraine of launching drone attacks on the fleet earlier this month, after Ukraine started its long-awaited counteroffensive.
Ukraine has repeatedly said that it aims to take back Crimea.
On Thursday, Russia said that Ukrainian missiles hit a bridge that connects Crimea to parts of Ukraine's Kherson region that is also currently controlled by Russia.
Russia uses the bridge to bring military equipment from Crimea into other areas under its control, The Washington Post reported.