Russia bombarded Ukraine’s second-largest city with massive strikes in the early hours of Saturday, its mayor said, killing at least three people, one night after Moscow carried out one of the war’s largest aerial assaults on Ukraine.

Ukraine said its “Operation Spiderweb” attacked Russian warplanes worth hundreds of millions of dollars, utilizing drones that each cost under $1,000 and were launched from wooden containers carried on trucks.

With one surprise attack after another, Ukraine keeps inventing new ways to wage war with drones. In turn, Russia is building a massive drone army of its own.