The drones targeted the Syzran oil refinery, more than 800km inside Russia, Zelenskiy said on social media, where he posted a video of the aftermath.
The governor of Russia's Samara region, Vyacheslav Fedorishchev, said two people were killed by Ukrainian drones in Syzran but he did not mention the refinery.
Russia's Astra news outlet said Ukrainian drones struck the Syzran refinery owned by oil and gas giant Rosneft.
Ukraine has expanded its mid- and long-range strike capabilities, deploying eye-catching drone and missile technology that it has developed domestically as it battles to defeat Russia's four-year-old invasion.
Ukrainian weaponry and expertise are now sought by other countries, whereas earlier in the war Kyiv had to plead for massive foreign military aid.
Ukrainian drones hit another refinery the previous day, Zelenskiy said, as attacks on Russian oil assets that play a key part in funding the invasion have become almost daily occurrences.
"Overall, our long-range plan for May is being carried out largely in full," Zelenskiy said in a social media post late on Wednesday.
"The key targets are Russian oil refineries, storage facilities, and other infrastructure tied to these oil revenues."
The escalating attacks have hurt Moscow's revenue at the same time as it feels the economic pinch of international sanctions.
With some attacks reaching more than 1500km into Russian soil, the strikes have contributed to some Russians feeling unsafe due to the war and heaped pressure on Russian President Vladimir Putin.
Ukraine's new reach has also helped it push Russian troops back along parts of the front line, with Ukrainian forces making their most significant battlefield gains since 2024, according to the Institute for the Study of War.
"Ukraine's intensified mid-range strike campaign against Russian logistics, military equipment, and manpower since early 2026 has also degraded Russian forces' ability to conduct offensive operations across the theatre and has also likely supported recent Ukrainian advances," the Washington-based think tank said in an assessment.
Russia's defence ministry said that air defences downed 121 Ukrainian drones between late on Wednesday and early on Thursday.
In the Belgorod region that borders Ukraine, eight people were injured by Ukrainian drones, according to the regional governor, Alexander Shuvayev.
Russia has also invested heavily in drones, using them to bombard civilian areas of Ukraine throughout the war and killing more than 15,000 civilians, according to the United Nations.
Ukraine's air force said it shot down 109 out of 116 drones that Russia launched overnight.
One civilian was killed and at least six others were wounded in the strikes in the north, south and east of the country, emergency services said.
