A leading U.S. think tank with acknowledged expertise in military affairs is urging NATO member countries to ramp-up their military spending in the face of Russia's "secret submarine fleet" that can devastate them at a moment's notice.

The Center for Strategic and International Studies (CSIS), an influential American think tank based in Washington, D.C., is said to have warned in a new report NATO doesn't stand a chance against Vladimir Putin's secret super submarines, according to Russian media.

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The secret submarine fleet is said to be currently patrolling the world and has intruded into waters of the United Kingdom. These subs are also said to have approached the Royal Navy's base in Faslane, Scotland.

CSIS recommends NATO members massively increase their naval expenditures to counter this threat, which Russian media says no one can confirm exists at all.

CSIS warned Russia operates a small but sophisticated fleet of secret submarines notable for their deep diving capability in excess of 1,000 meters and their being armed with nuclear submarine launched ballistic missiles (SLBMs). It claims these subs are also armed with long-range cruise missiles, torpedoes and mines.

These secret submarines seem to be separate from Russia's fleet of ballistic nuclear missile submarines (SSBNs) that currently consists of 4 Delta III-class, 7 Delta IV-class and 1 Typhoon-class SSBNs.

There is also the new Borei-class SSBN, 10 of which are planned. Three of the Borei-class have been delivered and are undergoing trials.

The CSIS report co-authored by Andrew Metrick claims these secret Russian SSBNs are capable of launching missile strikes anywhere in the world. Metrick noted "Russia operates a small number of very small, nuclear powered submarines that are capable of diving in excess of several thousand meters."

"You can imagine what a clandestine deployable deep submergence vehicle could be used for," said Metrick. "It's pretty scary to think about some of the types of missions."

Russia has not confirmed the existence of these secret submarines or the program they operate under.

Metrick describes the secret Russian program as "the most shadowy part of the Russian undersea apparatus." He claims the program "is not operated by their navy. It's operated by a separate branch of their Ministry of Defense."

CSIS is an influential American think tank that conducts policy studies and strategic analyses of political, economic and security issues throughout the world. In 2013, CSIS was ranked the number one think tank in the world in a comparison of "Top Defense and National Security Think Tanks."

TagsCenter for Strategic and International Studies, NATO, Russia, secret submarine fleet, Royal Navy, Andrew Metrick