British military spies on June 23 said Russia appears to be training combat dolphins in the annexed Crimean peninsula to counter Ukrainian forces.
In its latest update on the conflict, UK Defence Intelligence said the Russian Navy had invested heavily in security at the Black Sea Fleet's main base at Sevastopol since last year.
"This includes at least four layers of nets and booms across the harbour entrance. In recent weeks, these defences have highly likely also been augmented by an increased number of trained marine mammals," it added.
"Imagery shows a near doubling of floating mammal pens in the harbour which highly likely contain bottle-nosed dolphins."
The animals were "likely intended to counter enemy divers", it added.
The Russian Navy has used Beluga whales and seals for a range of missions in Arctic waters, the update said.
A harness-wearing whale that turned up in Norway in 2019, sparking speculation it was being used for surveillance, reappeared off Sweden's coast last month.
Norwegians nicknamed it "Hvaldimir" -- a pun on the word "whale" in Norwegian (hval) and a nod to its alleged association with Russia.
Hvaldimir's harness had a mount suitable for housing an action camera, and the words "Equipment St. Petersburg" printed on the plastic clasps.
In 2016, Russia's defence ministry sought to buy five dolphins as part of attempts to revive its Soviet-era use of the highly intelligent cetaceans for military tasks.
Both the Soviet Union and the United States used dolphins during the Cold War, training them to detect submarines, mines and spot suspicious objects or individuals near harbours and ships.
A retired Soviet colonel told AFP at the time that Moscow even trained dolphins to plant explosive devices on enemy vessels.
They knew how to detect abandoned torpedoes and sunken ships in the Black Sea, said Viktor Baranets, who witnessed military dolphin training in the Soviet and post-Soviet eras.
The US Navy used sea lions deployed to Bahrain in 2003 to support Operation Enduring Freedom after the 9/11 attacks in New York and Washington.
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