Russia's second-largest oil refinery, Kirishi, halted processing on Tuesday after Ukrainian drone attacks damaged three of its four crude distillation units (CDU), two industry sources told Reuters.
In an escalation of long-range drone attacks, Ukraine has pummelled Russia's oil industry for the past two months, from pipelines and ports to refineries and tankers, in an effort to undermine Russia's $3 trillion war economy.
Both Russia and Ukraine use swarms of long-range drones to try to smash vital infrastructure far behind the front lines - areas which are now being monitored around the clock by shorter-range drones.
Leningrad Governor Alexander Drozdenko said on Tuesday that the Kirishi refinery - known formally as Kirishinefteorgsintez - had been targeted and that there was a fire at an industrial zone in the town of Kirishi.
Two sources, who spoke to Reuters on condition of anonymity due to the sensitivity of the situation, said that three of the four crude distillation units of the refinery were damaged in the drone attack.
The CDUs are a fundamental part of any refinery and without them, the refinery cannot work.
Surgutneftegaz, which controls the refinery, did not immediately respond to a request for comment.
Ukraine struck the Kirishi refinery, Ukraine's security service said. Russia casts such attacks as terrorism. Ukraine says it is defending itself.
The Kirishi refinery, about 800 kilometres from the Ukrainian border, has suffered several Ukrainian drone attacks this year.
The sources said it was hard to estimate the time required for repairs of the damaged units, adding that several secondary units were also damaged.
The refinery's capacity is 20 million tonnes, or about 400,000 barrels per day. It has processed roughly 18 million tonnes annually in recent years, accounting for about seven per cent of Russia's oil refining volumes and is one of the key suppliers of diesel to the domestic market and for export.
(Reporting by Reuters; Editing by Guy Faulconbridge and Emelia Sithole-Matarise)