The unmanned stealth torpedo is powered by a nuclear reactor and intended by Russian designers to sneak past coastal defences on the seabed.
Russian defence officials said the device could deliver a multiple megatons warhead which would spark radioactive waves rendering vast areas of the target coastline uninhabitable for decades.
Christopher Ford, former US assistant secretary of state for International Security and Non-Proliferation, said the Poseidon was designed to “inundate US coastal cities with radioactive tsunamis”.
Security analysts agree the Poseidon 2M39 is “very real” and already coming to fruition.
Vladimir Putin has demanded regular updates on the progress of the missile’s sea trials from his defence minister Sergei Shoigu.
Norwegian intelligence chief Vice Admiral Nils Andreas Stensønes described the Poseidon as “part of the new type of nuclear deterrent weapons”.
He said: “It is in a testing phase. But it’s a strategic system and it’s aimed at targets and has an influence far beyond the region in which they test it currently.”
Satellite images appear to show a continuous build-up of Russian military bases and hardware on the country’s Arctic coastline, together with underground storage facilities likely for the Poseidon and other new high-tech weapons.
The Russian hardware in the High North area includes bombers and MiG31BM jets, and new radar systems close to the coast of Alaska.
Meanwhile, Kremlin defence sources said the Belgorod special-purpose nuclear submarine will enter state trials next month.
The Belgorod will carry Poseidon nuclear drones and the AS-15 deep-sea nuclear station.
An insider said it will serve in the Pacific Ocean after passing state trials and commissioning but will be able to perform missions in any location of in the world.