Ukraine has cultivated a network of agents and sympathizers inside Russia working to carry out acts of sabotage against Russian targets and has begun providing them with drones to stage attacks, multiple people familiar with US intelligence on the matter told CNN.

US officials believe these pro-Ukrainian agents inside Russia carried out a drone attack that targeted the Kremlin in early May by launching drones from within Russia rather than flying them from Ukraine into Moscow.

It is not clear whether other drone attacks carried out in recent days -- including one targeting a residential neighborhood near Moscow and another strike on oil refineries in southern Russia -- were also launched from inside Russia or conducted by this network of pro-Ukrainian operatives.

But US officials believe that Ukraine has developed sabotage cells inside Russia made up of a mix of pro-Ukrainian sympathizers and operatives well-trained in this kind of warfare. Ukraine is believed to have provided them with Ukrainian-made drones, and two US officials told CNN there is no evidence that any of the drone strikes have been conducted using US-provided drones.

Officials could not say conclusively how Ukraine has managed to get the drones behind enemy lines, but two of the sources told CNN that it has established well-practiced smuggling routes that could be used to send drones or drone components into Russia where they could then be assembled.