Ukraine attacked the Orsk refinery - about 1,300 kilometres from the front line - earlier this week, triggering a massive fire.
The governor of the Orenburg region, where the refinery is located, said on Thursday that the plant had been shut down and that repairs would take several months.
"The equipment is imported, and given the sanctions, repairs will take up to six months," Governor Yevgeny Solntsev said on social media.
"We are preparing for the worst-case scenario," he added.
Kyiv has been targeting Russian energy facilities with drone and missile attacks for months, hoping to strike at a key source of income for the Russian army.
