Ukraine's military said it had hit Tatneft's TANECO oil refinery in the Tatarstan region, about 800km east of Moscow, adding that a fire was recorded at the facility.
Russian investigators said they had opened a criminal case into what they qualified as a terrorist attack, saying that Ukraine had struck residential areas and that a child was among those civilians killed.
Russian Foreign Ministry spokesperson Maria Zakharova said the strike had hit industrial facilities and residential buildings and accused Kyiv of deliberately attacking civilians.
Ukraine and Russia have repeatedly said they do not target civilians.
Seven Uzbek citizens were among those killed in Nizhnekamsk, the Uzbek consulate in Kazan said.
A Tajik citizen was also killed, Russian state news agency TASS cited the Tajik foreign ministry as saying.
Nine out of 13 victims were in a hostel at the time of the strike, Russian state news agency RIA said, citing the press office of the head of the region. Reuters could not immediately verify the report.
In videos verified by Reuters on social media, smoke can be seen rising over the oil refinery.
The TANECO refinery, which was also hit by Ukraine in early June, processed 17 million metric tons of crude oil in 2024, producing 2.7 million tons of gasoline and 8.5 million tons of diesel fuel.
Ukraine has stepped up its strikes on Russian oil refineries in recent months in a campaign that has caused fuel shortages in many parts of Russia, although authorities say that most of those initial problems have since been resolved.
Ukraine says it wants to bring the price of Russia's war home to ordinary Russians.
Moscow has imported fuel from overseas to help bolster supplies.
On Sunday, both sides again accused each other of lethal attacks.
Three people were killed and 37 wounded in a Russian attack on the eastern Ukrainian city of Kharkiv, where a multi-storey apartment block was hit, while authorities in Russia's Belgorod said five were killed and 25 wounded by a Ukrainian drone attack.
