Fire broke out at an oil refinery in northwestern Russia on Sunday, resulting in deaths and injuries, local officials said.

The regional governor said the fire was not caused by a Ukrainian drone strike and investigators opened a criminal case on suspicion of negligence.

The fire near the city of Ukhta in Russia’s northwestern Komi Republic left at least three people injured, Komi’s emergencies ministry said. Regional investigators said that the fire also caused deaths, but did not specify how many. They did not say whether the fire had yet been extinguished.

“Today a fire occurred at an oil refinery facility in the city of Ukhta during scheduled technical work by a contractor, resulting in deaths and injuries,” the investigative department wrote on Telegram.

Investigators have opened a criminal case into possible negligence at the oil refinery.

Regional Gov. Vladimir Uyba said that the fire was not related to a drone strike. In recent months, Russian refineries and oil terminals have become priority targets of Ukrainian drone attacks, part of stepped-up assaults on Russian territory.

Local officials did not specify which company the refinery belongs to, but images published by the local emergencies ministry show the logo of oil giant Lukoil.