A major oil refinery in Russia’s Yaroslavl region is on fire after a big Ukrainian drone attack and emergency services are working to put out the blaze, Mikhail Evrayev, the regional governor, said on Thursday.

Unverified images posted on social media showed plumes of thick black smoke rising over the facility.

Evrayev said in a statement on Telegram that the refinery’s storage tanks had been struck by falling drone debris in what he described as the biggest enemy attack on Yaroslavl, some 250 km (160 miles) northeast of Moscow, so far.

“Today the region repelled the largest attack by enemy drones. All 93 drones were shot down by air defence and electronic warfare units,” said Evrayev. He said four people had received shrapnel wounds, but that nobody had been killed.

Ukrainian President Volodymyr Zelenskiy said on Telegram earlier on Thursday that the Ukrainian military had hit two Russian oil refineries – the Bashneft-Novoil refinery in the republic of Bashkortostan, and the Slavneft-Yanos refinery in the Yaroslavl region. There was no immediate comment from Russian officials in Bashkortostan.

Ukraine has stepped up its attacks on Russian oil refineries in recent months, causing fuel shortages which the authorities have stabilised in many parts of Russia though government officials say problems remain in some areas.

The refinery in Yaroslavl – which has been targeted by Ukraine before – has a processing capacity of 15 million metric tons per year, or around 300,000 barrels per day.