THE Pentagon said it is "very closely" watching Russia's military build-up in the Arctic as Vladimir Putin presses ahead with tests of a doomsday nuclear torpedo that could wipe out New York.
The Kremlin is said to be planning to deploy dozens of Poseidon 2M39 underwater drones - designed to sneak up on cities and unleash a radioactive tsunami.
The terrifying weapons have been developed as Putin steps up his claim over the strategic territory around the Pole - which could yield huge oil and gas reserves.
Satellite pictures this week revealed Russia is busy refitting Cold War-era nuclear bases on Franz Joseph Land, a group of islands in the Arctic Ocean.
Analysts say Russia is also beefing up its military presence on the Kola Peninsula near the city of Murmansk, close to the border with Nato member Norway.
It is building underground storage facilities that could be intended for the Poseidon torpedoes and other high-tech doomsday weapons.
Military jets and bombers are also pictured as well as new radar systems.
“We’re monitoring it very closely,” said Pentagon spokesman John Kirby.
“The Arctic is key terrain that’s vital to our own homeland defence.”
And a senior US State Department official told CNN: “There's clearly a military challenge from the Russians in the Arctic.
“That has implications for the United States and its allies, not least because it creates the capacity to project power up to the North Atlantic.”
Reports in Russia said the defence ministry plans to deploy 30 Poseidon missiles at the Northern Fleet's coastal facilities.
The nuclear-powered torpedoes have a reported range of 6,200 miles and are said to be capable of creeping along the ocean floor to evade coastal defences.
Once close to shore, a nuclear blast would trigger a tsunami that would flood a city such as New York with radioactive waves, rendering it uninhabitable for decades.
Many observers thought the scheme was fantasy when Putin announced it in 2018.
But pictures released by the defence ministry this week are said to show advanced tests under way in the Arctic.
The president asked top brass for an update on a “key stage”’ of the Poseidon tests in February, and more testing is due this year.
Last November, Christopher A Ford, then US assistant secretary of state for International Security and Non-Proliferation, said Poseidon is designed to "inundate US coastal cities with radioactive tsunamis."
Russia is also testing hypersonic Zircon anti-ship cruise missiles in the Arctic, state media says.
Footage released by the Kremlin in December boasted the 6,000mph rockets were "truly unparalleled in the world" after a direct hit on a target hundreds of miles away.
Russia's sabre-rattling has pushed tensions in the Arctic region to the highest in decades.
The Kremlin's military build-up has been seen as an effort to tighten its grip on the region as melting ice opens up the possibility of new shipping routes and oil drilling.
Last year a group of US warships including a missile destroyer sailed into the Barents Sea off Russia’s northern coast for the first time since the collapse of the Soviet Union.
Washington said the exercises were aimed at promoting “regional security and stability”.
Last month three Russian nuclear-powered subs simultaneously broke through Arctic ice in an exercise aimed at showing off the nation’s military prowess.
It comes amid growing fears of all-out conflict between Russia and neighbouring Ukraine, seven years after the start of the civil war and Putin's annexation of Crimea.
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Russia has sent 4,000 troops and convoys of tanks to the border of the breakaway Donetsk region, which is held by rebels.
Experts have warned Ukraine's allies could be dragged into a new world war within weeks.
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And astonishing claims on Russian state TV this week suggested Putin should consider firing nukes if Nato did not back down on its demands.