Ukraine has “cultivated” one network of agents and supporters inside Russia working to carry out sabotage against Russian targets, and has begun providing them with drones to carry out attacks, CNN reveals, citing multiple sources familiar with US intelligence on the matter.

US officials believe these pro-Ukrainian operatives inside Russia carried out the attack with drone that targeted the Kremlin in early May, launching the drones from inside Russia, instead of flying them from Ukraine to Moscow.

It is unclear whether other drone attacks in recent days – including one targeting a residential area near Moscow and another attack on oil refineries in southern Russia – were also launched from inside Russia, or whether the attacks were carried out by this pro-Ukrainian network.

However, US officials believe that Ukraine has develop sabotage cores within Russia consisting of pro-Ukrainian supporters well trained in this kind of warfare. Ukraine is believed to have supplied them with Ukrainian-made drones, and two US officials told CNN there is no evidence that any of the drone strikes have been carried out using US-supplied drones.

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

A European intelligence official also noted to the US network that the Russia-Ukraine border is huge and very difficult to control, making it “ripe” for smuggling — something the official commented the Ukrainians have been doing for most of the decade they’ve been in. war with pro-Russian forces.

Exactly who controls these people is also unknown, the sources told CNN, although US officials believe people within Ukraine’s intelligence service are involved. Ukrainian President Volodymyr Zelensky has set broad parameters for what his intelligence and security services are allowed to do, two of the sources said, but not every business is required to sign off on him.

The head of Ukraine’s military intelligence, Kirill Budanov, has consistently proposed some of the most brazen plans for operations against Russia and appreciates the value of token blows, US officials told CNN.

Classified Pentagon documents leaked online earlier this year revealed that the CIA urged Budanov to “postpone” attacks against Russia on the anniversary of its invasion of Ukraine, according to the Washington Post. Butanov agreed to the CIA’s request, the classified documents reportedly state. But the drones were spotted near Moscow on February 28, just days after the one-year anniversary of the war.

Another leaked US intelligence report obtained by CNN, which comes from the Signals Intelligence Service, says that Zelensky in late February “suggested striking Russian development sites in Russia’s Rostov region” using drones aircraft, as Ukraine does not have long-range weapons capable of reaching that far.

It is unclear whether that plan went ahead, but oil facilities in the Rostov region have caught fire after being hit by suspected drones several times in the past year – attacks that Russia is now investigating and has blamed on “criminal actions by its armed forces” of Ukraine”.

“All I will comment on is that we are killing Russians,” Budanov told Yahoo News last month when asked about the car bomb attack that killed the daughter of a prominent Russian political figure in suburban Moscow last year. The US intelligence community estimated that this operation was approved by elements within the Ukrainian government.

“And we will continue to kill Russians anywhere in the world until the complete victory of Ukraine,” Budanov said.