Ukraine kept up its heavy drone assault on Russia, setting fire to a major oil refinery in the south and killing at least two people, Russian authorities said on Sunday, as President Vladimir Putin acknowledged his country is going through a “difficult period”.

Ukraine has markedly stepped up its long-range attacks on Russian military industries and energy facilities in recent months, aiming to cut Moscow’s revenue for its invasion – now in its fifth year – and make Russians feel the consequences.

The campaign has choked Russian fuel supplies and military deliveries. According to Western analysts, it has also slowed Moscow’s efforts on the battlefield, heaping pressure on the Kremlin to come to the negotiating table.

“Our ‘long-range sanctions’ reached two oil refineries in Russia,” Ukrainian President Volodymyr Zelensky wrote on the Telegram messaging app on Sunday. “Each [strike] means a reduction in the resources that fuel the Russian war machine, and another step toward peace.”

Debris from downed Ukrainian drones sparked a blaze at the refinery in Slavyansk-na-Kubani, a town in Russia’s Krasnodar region, east of occupied Crimea, according to Governor Veniamin Kondratyev. The falling debris killed one person in Slavyansk and wounded another in a nearby village, local authorities said.

The Slavyansk site is one of southern Russia’s major refineries, processing close to 4 million tons of crude per year, according to its operator’s website. It is also a key source of petroleum products intended for export through Russia’s Black Sea ports, including fuel oil, naphtha and marine fuel.