Ukraine's almost daily long-range drone and missile strikes using domestically developed weapons have battered Russian oil facilities for months.
The operation has brought a fuel crisis that is embarrassing the Kremlin more than four years after Russia's all-out invasion of its neighbour.
The overnight attacks started fires at a Lukoil refinery in Russia's Perm region, more than 1500km from Ukraine, and a refinery in Ryazan, about 400km from the border, Ukraine's Security Service and General Staff said on Wednesday.
The refineries have a combined annual production capacity of around 30 million tonnes of oil, according to Ukraine, and make a vital contribution to Russia's military effort and its government revenue.
Ryazan governor Pavel Malkov said only that six people were taken to hospital after a drone attack.
He said drone debris caused fires at unidentified "industrial sites".
Russian online retailer Wildberries, a major supplier likened to Amazon that Ukraine has targeted in recent days, said it evacuated employees from a facility in Ryazan.
Russia has sophisticated air defences but its vast territory is hard to comprehensively cover.
The attacks came hours after President Volodymyr Zelenskiy visited Washington on Tuesday for talks with US President Donald Trump to push for more vital American help for Ukraine's fight against its much bigger enemy.
"Russia must feel that every day of this war will only come at a higher cost for it. We need to weaken the aggressor and keep up the pressure to bring an end to this war closer," Zelenskiy said on social media.
Ukraine's recent high-profile successes against Russia have impressed Washington and helped restore US goodwill after Trump's reluctance at the start of his second term to keep supporting Kyiv.
That stance had raised the spectre of Ukraine and Europe having to stand up to Russian aggression alone.
Zelenskiy said he had a "good meeting" with the American president, and Trump described the meeting as a "Great Honor".
"Many things were discussed. The meeting went very well!" Trump wrote on Truth Social.
Zelenskiy said he also had a meeting in Washington with representatives of both parties in the US Senate, with more than 60 senators in attendance, as Ukraine pushes for more sanctions on Russia.
He also met representatives of Lockheed Martin, days after he welcomed a delegation from Raytheon to Kyiv.
The US weapons manufacturers are a crucial source of sophisticated weaponry, such as Patriot air defence systems, that Ukraine hopes to make itself.
Trump said earlier in July he would grant Ukraine licences to manufacture the Patriots it needed to counter Russia's ballistic missiles.
Prospects for a settlement to stop the fighting appear dim, despite a yearlong diplomatic push by the Trump administration.
But US officials also agreed that Steve Witkoff and Jared Kushner, the president's main envoys, would make a trip to Kyiv sometime in the future, according to a person familiar with the talks.
Major General Mykhailo Drapatyi, the new commander-in-chief of Ukraine's military, said on Wednesday that he had held his first phone conversation with General Alexus G Grynkewich, the supreme allied commander Europe and commander of US European Command.
"I outlined my vision of Ukraine's victory and the capabilities required to achieve it," said Drapatyi, who was appointed last week as part of a government reshuffle.
Ukrainian units also have struck 201 Russian vessels in the past three weeks in the Black Sea and Sea of Azov, which border both countries, a senior military official said.
Ukrainian forces also struck a naval base used Russia's Black Sea, according to the General Staff.
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