KYIV — The Security Service of Ukraine sent a barrage of drones to attack four military airfields in Russia on Sunday, hitting 41 of Moscow’s strategic bombers, an official with the service told POLITICO.

The daring attack came the day before the next round of Russia-Ukraine negotiations is set to take place in Istanbul.

“Russia’s military planes are currently burning at the Belaya, Diaghilev, Olenya and Ivanovo airfields,” said the official from the service, also known as the SBU.

SBU chief Vasyl Malyuk “personally curated the operation,” the official told POLITICO, speaking on condition of anonymity to discuss military operations.

“Thanks to Malyuk … Russia has now understood the true meaning of the word demilitarization,” the official said.

The official showed a video of Russian bombers burning at one of the airfields that could not be independently verified. Russian authorities have not issued any official statements on the drone attacks.

“Today will later be called the dark day of Russia’s long-range aviation,” Russian military aviation blogger Ilya Tumanov, known as Fighterbomber, said in a Telegram post. “And the day is not over yet.”

The SBU official said that to prepare for the operation, called “Pavutyna”(web), Ukrainian operatives delivered FPV drones to Russia territory along with mobile wooden housings.

“Later, in the Russian Federation, the drones were hidden under the roofs of housings, already placed on trucks. At the right moment, the roofs of the housings were opened remotely, and the drones flew to strike the Russian bombers,” the official said.

The official said all people involved in the complex logistical operation were already safely back in Ukraine.

Russia has been using Olenya and other airfields to attack Ukraine with missiles and drones.

On Sunday, a Russian missile hit one of the training grounds of Ukraine’s Land Forces, killing 12 people and injuring more than 60 others, the Land Forces said in a statement.

 “To clarify all the circumstances and causes of the personnel losses, a commission has been created and an internal investigation has been appointed at the Land Forces Command of the Armed Forces of Ukraine. The command and personnel of the eastern operational command will also provide full assistance to the work of law enforcement agencies,” the statement said.