Russian-backed authorities in the occupied Crimean city of Sevastopol said on Friday that an overnight attack by Ukraine’s military had hit an electricity substation, leading to rolling blackouts throughout the city that is home to part of the Russian naval fleet.

Schools have been closed across Sevastopol, a city of about a half million people on the Black Sea, Mikhail Razvozhaev, the Russian-backed governor of the city, said on Telegram. At least 51 drones were intercepted overnight in Crimea, Russia’s Ministry of Defense said. Ukraine’s military did not immediately comment on the attacks.

Reports on social media showed fires and explosions, including at a fuel depot in the city of Novorossiysk. Internet connectivity in Sevastopol dropped to about 16 percent, according to NetBlocks, an internet monitoring group.

Ukraine’s military has made some of its most significant progress with its operations in the Black Sea. After Russia withdrew from an agreement allowing Ukraine to resume some export shipments through the sea, Ukraine’s military began a campaign to drive out the Russian Navy. It destroyed many of Russia’s warships and attacked its headquarters.

The operation allowed Ukraine to establish a new shipping corridor, and seaborne grain and oilseed exports are nearing prewar levels, according to data shared with The New York Times.

This a developing story.