Ukrainian secret services were able to attack strategic bomber aircraft at Russian air bases on Sunday by hiding explosive-laden drones inside the roofs of wooden sheds, according to a Ukrainian security official and images posted online.
Ukraine's domestic security agency, the SBU, acknowledged that it carried out the operation, codenamed "Spider's Web" and said it had caused considerable damage.
The sheds were loaded onto trucks that were driven to the perimeter of the air bases. The roof panels of the sheds were lifted off by a remotely-activated mechanism, allowing the drones to fly out and begin their attack, the official said.
The security official, who spoke on condition of anonymity, said strikes were conducted on Sunday on four air bases, and that 41 Russian warplanes were hit.
An SBU statement posted on the Telegram messaging app estimated the damage caused by the assaults at $7 billion.
"Thirty-four percent of strategic cruise missile carriers at the main airfields of the Russian Federation were hit," the SBU said on the Telegram messaging app.
President Volodymyr Zelenskiy, writing on Telegram, expressed delight at the "absolutely brilliant outcome".
"And an outcome produced by Ukraine independently," he wrote, noting that the operation had taken more than a year and a half to prepare. "This is our longest-range operation."
Speaking shortly afterwards in his nightly video address, the president noted that 117 drones had been used to attack the Russian bases and that Russian forces suffered "very tangible losses, and justifiably so".
Zelenskiy said the SBU had set up a nerve centre for the operation right next to a regional office of Russia's FSB intelligence service. All operatives taking part had been brought out of Russia "on the eve of the operation", he said.
Video shows bombers ablaze
Unverified video and pictures posted on Russian social media showed Russian strategic bombers on fire at the Belaya air base in the Irkutsk region of Siberia.
Igor Kobzev, the regional governor, said there had been a drone attack on a military unit near the village of Sredny, which is near the Belaya base, though he did not specify what the target was. He said the drones had been launched from a truck.
The Irkutsk region attack was the first time a drone assault had been mounted by Ukraine so far from the front lines, which are more than 4,300 km (2,670 miles) away.
That is beyond the range of the long-range strike drones or ballistic missiles Ukraine has in its arsenal, so required a special scheme to get the drones close enough to their targets.
Photographs shared with Reuters by the Ukrainian security official showed dozens of short-range quadrocopter drones piled up in an industrial facility. The official said these were the same devices used in the attack.
Other images shared by the official showed the wooden sheds with their metal roofing panels removed, and the drones sitting in the cavities between roof beams.
Separate video posted on Russian Telegram channels, which has not been verified by Reuters, appeared to show matching sheds on the back of a truck.
The roof panels can be seen lying on the ground next to the truck, and the video footage shows at least two drones rising out of the top of the sheds and flying off.
The Russian online media outlet that posted the video, Baza, said in a caption that it was filmed in the district near the Belaya air base.
The Irkutsk region air base hosts Tupolev Tu-22M supersonic long-range strategic bombers, a type of aircraft that has been used to launch missiles against targets in Ukraine.
The operation, according to the Ukrainian security official, was personally overseen by Zelenskiy and Vasyl Maliuk, head of the SBU domestic intelligence agency.
If confirmed, the strikes would be the most damaging Ukrainian drone attack of the war, and would be a significant setback for Moscow.
The source shared video footage shot from a drone, saying it showed one of the strikes. The images showed several large aircraft, some of which appeared to be Tu-95 strategic bombers, on fire.
