Small, maneuverable and impossible to jam, Russia’s best fiber-optic drones can—in the hands of skilled operators—slip into the buildings and dugouts sheltering Ukrainian vehicles. A video circulated by Russian sources on or before Monday depicts a warhead-laden quadcopter drone easing under a tarp and into a barn where a Ukrainian Bohdana wheeled howitzer was parked.

Russia said it was hit by a “massive” drone attack overnight Tuesday, hours ahead of critical talks between officials from Washington and Kyiv.