Dolphins are being used to protect a Russian navy base in the contested Crimea, military experts believe, as bizarre new satellite images emerge.
The photos taken from above appear to show dolphin pens at the harbour of Sevastopol, Crimea's largest city and a major port which looks out to the Black Sea.
The images have sparked concern that the sea life is being militarised to block Ukrainian operations.
The mammals are being used to deter underwater missions by detecting swimmers and mines, according to a US Naval Institute report.
The satellite images have been published by Maxar Technologies, which has provided photos of the war since Russia's invasion of Ukraine in February.
The USNI report said it was unclear whether the Ukraine military intends to sabotage the base, but explained how marine mammal programmes train animals to perform protection tasks.
"This could prevent Ukrainian special operations forces from infiltrating the harbour underwater to sabotage warships," the report said.
The US has a Mark 7 Marine Mammal System, better known as a bottlenose dolphin, to detect sea mines, Forces News reports.
Before Russia's invasion Gervase Phillips, a lecturer in history, politics and philosophy at Manchester Metropolitan University, spoke to Forces News about the role of militarised animals.
He said: "We have nothing like a dog's nose to smell explosives yet and nothing as good at finding things on the seabed as a dolphin or a sea lion.
"There are all kinds of those military roles for which, unfortunately, we have no practical alternative."
The US Navy began its marine mammal programme in 1960, originally hoping to both improve the hydrodynamics of its torpedoes, and its ability to detect objects under water, by studying dolphins.
Yet the scope of this programme appeared to expand rapidly.
Dolphins were soon being trained to locate enemy mines and lost objects on the seabed.
According to the Manchester Metropolitan University report, the Soviet Union is believed to have trained dolphins in a similar manner to the Americans at a facility in Crimea on the Black Sea.
In 2000, the BBC reported that many of these dolphins were sold to Iran following the collapse of the Union.
The facility was re-opened by the Ukrainian Navy in 2012, but since the Russian annexation of Crimea in 2014 has been back in the hands of the Russian Navy.
