Russian authorities said on Saturday that 26 people had died from alcohol poisoning this week after consuming locally-produced spirits in the Orenburg region bordering Kazakhstan.
The death toll rose from nine reported on Thursday, the regional ministry told RIA news agency.
Another 28 people are suffering alcohol poisoning symptoms, it said.
The regional branch of the Investigative Committee of Russia has opened a probe to determine whether alcohol products sold in the region, some 1,500 kilometres (900 miles) southeast of Moscow, meet safety standards.
On Friday, the committee said that six people had been detained for allegedly producing and selling alcohol unfit for consumption.
After a case of mass alcohol poisoning in Siberia in 2016, national authorities ordered tighter controls on the production and sale of beverages, medicines, perfumes and other liquids containing a high percentage of ethanol.