Russia launched an intense missile and drone barrage at the Ukrainian capital Kyiv in the early hours of Friday, killing at least six people, Ukrainian officials said, as powerful explosions reverberated across the country.

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.

The attack on Friday, which killed at least four people nationwide, came two days after Russia vowed retaliation for an audacious drone assault on its strategic bomber bases.