Russia is one of the world's biggest energy producers, with oil providing an economic mainstay as its army tries to push deeper into Ukraine in a slow and costly advance more than four years after Moscow launched its full-scale invasion.
The attacks have dented Russia's refining capacity, bringing shortages at petrol stations.
Kyiv officials say the onslaught seeks to compel Russian President Vladimir Putin to seek a peace deal.
There is no sign that strategy is working.
Ukraine's General Staff said its forces struck the Gazprom Neftekhim Salavat oil refining and petrochemical complex in Russia's republic of Bashkortostan overnight, causing a fire at the facility.
The complex, southeast of Moscow about 1300km from Ukraine's border, is one of Russia's largest oil refining and petrochemical facilities, the General Staff said in a Telegram post on Thursday.
The plant processed up to 74 million barrels of oil annually, producing petrol, diesel fuel and other products, it said.
Russia's Bashkortostan governor, Radiy Khabirov, said only that a drone attack wounded two civilians and started a fire in an industrial zone of Salavat.
Wildberries, Russia's biggest online retailer whose giant warehouses have repeatedly been struck by Ukrainian drones, said a fire started at one of its facilities in the Salavat industrial zone after the overnight attack.
The company said the facility was evacuated but that goods were not being stored there.
The attack came a day after Ukrainian anti-ship missiles and drones blitzed a major Russian naval base on the Black Sea coast.
Elsewhere, four explosive experts from Russia's emergency ministry and a security guard were killed on Thursday after an unspecified Ukrainian weapon exploded in Sevastopol, a port city in illegally annexed Crimea, the Russian-installed governor of Sevastopol Mikhail Razvozhayev said.
They were inspecting the site of a Ukrainian attack when "an enemy weapon detonated", he said.
It was not possible to independently verify either side's reports.
Russia has responded by hammering Ukraine with ballistic missiles, jet-powered drones and glide bombs, all of which are powerful and hard to intercept.
The strikes have killed more than 16,000 civilians since the war began, according to the United Nations.
Russian attacks featuring 133 long-range strike drones killed at least four people and wounded eight others across Ukraine overnight from Wednesday to Thursday, officials said.
A jet-powered Shahed drone struck a passenger train in the southern Odesa region, killing the engineer and his assistant, Ukraine's state railway company, Ukrzaliznytsia, said on Thursday.
A Russian drone killed two men on Wednesday evening in the southern city of Kherson, regional military administration chief Oleksandr Prokudin said.
Ukraine desperately wants more US-made Patriot interceptor missiles that can counter Russia's ballistic missiles, but stocks of the air defence system reportedly are low because of the Iran war.
Ukraine so far in 2026 has two-and-a-half times fewer Patriot interceptors than in 2025, according to Ukrainian President Volodymyr Zelenskiy, saying he makes calls every day to try and source some more.
He said in an interview with CNN that he was asking the United States to sell five per cent of the interceptor missiles they had in stock to help Ukraine make it through next winter, when Russia's strikes focus on the power grid.
He has so far obtained just one per cent, he said.
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