The project, code named 20120, came to light when details were inadvertently posted on the web site of the Novgorod region town of Sarov, Kommersant said.

The original press release in which the details were released had been removed from the web site as of Wednesday, though a copy could still be accessed at the web portal Sarov-city.ru.

A woman who answered the phone at the Sarov city administration Wednesday said no one was available to comment, Reuters reported. A navy spokesman also declined to comment.

The specifications for the 20120 and the secrecy around it suggested that it contained technology that was radically different from any submarines in service, Kommersant reported. It said the design could involve installing a small nuclear reactor on a diesel-powered sub to extend the time it can operate without surfacing.

Earlier this year, Russia launched the Yury Dolgoruky nuclear submarine, its first new-generation sub since the end of Soviet rule, Reuters reported.