It comes as the Pentagon announced plans to quadruple its defence budget for projects in Europe, with US Defence Secretary Ash Carter describing Russia as a growing threat.
Top British naval officer Clive Johnstone, commander of NATO's Maritime Command, said Putin's U-boats possess "a level of Russian capability that we haven't seen before".
He said Russia had made "remarkable" leaps in technology "through an extraordinary investment path not mirrored by the West".
Royal Navy Vice Admiral Johnstone said there was "concern" that Russian submarines now "have longer ranges, they have better systems, they're freer to operate".
Speaking to defence magazine IHS Jane, he said NATO had also seen "a rise in professionalism and ability to operate their boats that we haven't seen before.
"None of that would worry us if we knew what the game plans were or we knew why they were deploying or what they were doing [but] we don't understand what the strategic and operational objectives are of the Russian state.
"A lot of what the Russians are doing at the moment we don't understand, and is obscure and is shrouded in other activity which makes us nervous, and makes nations nervous."Tensions between Russia and the West have been heightened since the annexation of Crimea in 2014.
The Kremlin's involvement in the war in Syria has also raised eyebrows, with claims that Putin's jets are targeting moderate militia groups backed by Britain and the US.
In response, Mr Carter has revealed Washington will boost the defence budget in 2017 to $3.4bn (£2.35bn) for its European Reassurance Initiative - up from $789m (£545m) this year.
But we will see what will happen
