NATO navies worry about those subs and they've increased their focus on countering undersea threats.
Nordic navies are investing in their own submarine fleets to keep track of Russia's boats.
A particular concern for the alliance is Russia's submarines, many of which are assigned to those two fleets.
The potential threat from Russia's undersea forces has prompted its neighbors to reevaluate their own submarine needs.
But Sweden's western neighbors, Norway and Denmark, both see a need for bigger sub fleets.
Persons:
Christopher Cavoli, OLGA MALTSEVA, Ronald Reagan, Fredrik Linden, Petty, Marlowe Dix, Michael Aastrup Jensen, Aastrup Jensen, HENRIK MONTGOMERY, Eirik Kristoffersen, Kristoffersen, Constantine Atlamazoglou
Organizations:
NATO, Nordic, Service, Baltic, US, Command, Allied, Getty, North Atlantic, Baltic Fleet, Navy, Submarine, Reuters, Naval, Norfolk, US Navy, Hudson Institute, Getty Images, Submarines, Armed Forces, Fletcher School of Law, LinkedIn, Twitter
Locations:
Ukraine, Wall, Silicon, Nordic, Gulf of Finland, St . Petersburg, AFP, Finland, North, Russia's, Kaliningrad, Russia, Baltic, Sweden, Swedish, Gotland, Blekinge, Navy Gotland, Sweden's, Norwegian Ula, Norway, Denmark, Danish, Ula, Oslo, Swedish Gotland, Halland, Stockholm