The Russians are seeking five trainable dolphins willing to go into combat. The military is offering $24,000 (£17,000) for delivery of the dolphins to the Crimean port city of Sevastopol by 1 August, according to a document uploaded to the government's procurement website.

Officials are seeking two female and three male dolphins between the ages of three and five years old with perfect teeth and no physical impairments, reports Sky News. They aim to use the dolphins in military operations, a source told RIA Novosti state news.

Dolphins were enlisted by both the Soviet Union and US during the Cold War. The animals were trained to detect submarines, underwater mines and spot suspicious objects or individuals near harbours and ships.

Combat dolphins in the Soviet era were also trained to plant explosives on enemy vessels and knew how to detect abandoned torpedoes and sunken ships in the Black Sea, said retired colonel Viktor Baranets, who was familiar with the training.

Crimea has housed the dolphin training facility since 1965. But the training centre deteriorated after the collapse of the Soviet Union and its remaining dolphins were reportedly sold to Iran.

The Ukrainian navy reestablished the centre in 2012 but lost it — and its trained dolphins and seals — to the Russian navy when the nation annexed the area two years later.

The US navy continues to use sea mammals to carry out military operations. Sea lions were deployed to Bahrain in 2003 to support operations there after the 9/11 attacks.

In 2014 the US used 20 military-trained dolphins and 10 sea lions in NATO military exercises in the Black Sea. The animals were airlifted to the Ukraine.

The US at the time was developing new armour for dolphins to wear.