A freight train that derailed in Russia’s southern Volgograd region overnight did so due to “outside interference”, the regional filial of Russian Railways said on Tuesday.

The incident occurred at about 1am near the village of Kotluban, when a tanker transporting diesel exploded causing another carriage transporting lumber to catch fire, the Emergency Situations Ministry said. It took two hours to bring the fire under control.

No casualties or damage were reported, and inquiries into the cause of the incident were being made. Neither Volgograd Governor Andrey Bocharov nor the Russian Defence Ministry has commented on the incident, local news website V1.ru reported.

The ministry said repair work was ongoing and rail traffic had been restored shortly after 5am, though there were interruptions to passenger services between Volgograd and St. Petersburg.

Telegram channel Baza reported that the fire was caused by a drone attack. “The drone crashed into the fuel tank, which caused an explosion and a fire. Nine carriages left the tracks,” it said.

The incident comes amid an increase in Ukrainian drone attacks targeting civilian and energy infrastructure in southern Russia in recent months.