Explosives found in a residential building in St Petersburg are similar to the unexploded bomb in the city's metro, found on the day of the bombing that killed 14 people on Monday, Russian security sources said on Thursday.

"Explosives were in a similar amount that was used at Vosstaniya (metro station)," a security source told Reuters.

A Reuters witness near the searched building heard officials discussing that the explosive device "was exactly like" the unexploded bomb.