The prototype doomsday craft took to the waves for the first time on Christmas Day to begin under-sea trials, Russia's state Tass news agency reported.

A defence source is quoted as saying: "In the sea area protected from a potential enemy’s reconnaissance means, the underwater trials of the nuclear propulsion unit of the Poseidon drone are under way."

The drone is said to be powered by a miniaturised nuclear reactor, which has been fitted to the rear for ocean tests.

However development is at an early stage and the craft is being carried by one of the Russian navy's nuclear submarines "as part of experimental design work rather than full-fledged sea trials at this stage", the source told Tass.

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