Several Ukrainian regions suffered power outages on Sunday after Russia launched what the state grid operator called the “most massive strike” against Ukraine’s power plants since the beginning of Moscow’s full-scale invasion of the country.

Kyiv responded with a counterattack of drones overnight into Sunday, targeting energy infrastructure and leaving the Russian city of Voronezh and around 20,000 people without electricity, Reuters and AFP reported.

Ukraine’s grid operator said the Russian strikes hit its energy plants continually from Friday into Saturday, and the country’s generation capacity was “zero” on Saturday. The Russian assault included hundreds of drones and dozens of missiles.

“We lost everything we were restoring 24 hours a day! Every time the enemy strikes even more brutally, even more cynically,” the operator said in a post on Facebook.

The company scheduled power cuts that can last up to 16 hours in some regions, as it works to repair the power supply.

“Emergency power cuts have been introduced in a number of regions of Ukraine,” Energy Minister Svitlana Vasylivna Hrynchuk said on Telegram. They “will be canceled after the situation in the power system stabilizes.”

The main targets of the attack were the Kyiv, Dnipropetrovsk and Poltava regions, according to the Ukrainian air force.

The Russian strikes have targeted energy, heat and water supplies in many Ukrainian cities, as well as the Khmelnytskyi and Rivne nuclear plants, Ukraine’s Foreign Minister Andrii Sybiha said.

“Russia is deliberately endangering nuclear safety in Europe. We call for an urgent meeting of the IAEA Board of Governors to respond to these unacceptable risks,” Sybiha wrote on X.