Six of the submarines are undergoing repairs and upgrades at Zvezda shipyard at Bolshoy Kamen on Russia's Pacific coast.
According to navy and shipyard representatives, the upgrades include new missiles and other weapons. The work aims to put the submarines on the same technological level as Russia's next-generation nuclear-powered boats, such as the new Project 885M Yasen-class submarines.
The six boats reportedly include Schuka-B/Project 971 Akula-class nuclear-powered attack submarines (photo) and Antyey/Project 949A Oscar II-class guided-missile submarines that were built in the late 1980s and early 1990s.
Three of the six subs reportedly are receiving new anti-ship cruise missiles.
The modernization program has left current force levels as low as nine active submarines across the fleet.