A Ukrainian drone strike has forced a shutdown at one of Russia’s largest oil refineries, knocking out a key processing unit and cutting overall production by a quarter.

The attack targeted the Kirishinefteorgsintez plant in Kirishi, Leningrad Oblast, which is operated by energy giant Surgutneftegaz. The facility normally handles about 20 million tons of crude each year, making it a vital hub for Russia’s fuel supply.

According to sources, the drones struck equipment inside the plant, damaging a furnace and sparking a fire that was later contained. The affected unit is responsible for up to 40% of the refinery’s throughput, and repairs could take about a month.

As a result, the refinery is expected to run at only 75% of its nominal capacity. Operators are attempting to minimize disruption by increasing output from other sections of the complex.

Last year, the plant processed 17.5 million tons of crude, which accounted for roughly 6.6% of Russia’s overall refining volume. It produces a range of essential fuels, including gasoline, diesel, fuel oil, and bitumen, much of which is key for both domestic use and exports.

The strike follows one of the heaviest drone barrages on Leningrad Oblast earlier this month, when more than 30 UAVs were intercepted. Days earlier, Ukrainian drones also targeted the Bashneft-Novoil refinery in Ufa, triggering explosions and a fire.

The back-to-back refinery attacks highlight the growing pressure on Russia’s energy infrastructure, a sector central to its economy and funding of the ongoing war.