Overnight drone strikes were reported in the major oil-processing and petrochemical region of Russia, as well as at railway stations and electrical substations in Ukraine.
According to official comments, three people were injured in the Ukrainian strike on Russia's central Tatarstan region, while one railway worker was killed in the Russian attack in Ukraine's Sumy region.
Regional leader Rustam Minnikhanov stated on the Telegram messaging app that Tatarstan's industrial facilities were the target.
According to Mayor Radmir Belyayev, due to the possibility of drone attacks, the regional city of Nizhnekamsk will cancel public activities on Friday, when the country marks Russia Day with a national holiday and celebrations.
Belyayev did not mention any damage caused by the drones. The region is home to a number of significant industrial facilities, including Tatneft's oil refinery TANECO and Sibur's Nizhnekamskneftekhim petrochemical plant.
According to the regional governor, Togliatti, a city on the Volga River some 800 kilometres (500 miles) south-east of Moscow and the location of Russia's largest automaker, Avtovaz, was also attacked by drones.
On Thursday night, Acting Regional Governor Yegor Kovalchuk said that Ukraine's attack on Russia's border region of Bryansk killed two people and injured 10 others.
Both Russia and Ukraine deny intentionally attacking or targeting civilians.
News agencies, citing Russia's Defence Ministry, stated that Russia had so far downed 231 Ukrainian drones overnight.
The chief executive of Ukraine's state railway, Ukrzaliznytsia, said on Friday that Russian drones attacked substations, railway stations and electrical signalling posts in the country's northern Sumy region, killing a railway worker.
Another railway worker was injured, according to CEO Oleksandr Pertsovskyi. The incident came after an overnight attack on the town of Konotop, also in the Sumy region, on Thursday, which resulted in the death of a railway worker and left four others injured.
According to Ukraine's Air Force, Russia had launched 117 drones since Thursday, and 102 of them were downed or neutralised by air defence systems.
In the southern city of Mykolaiv, a drone strike occurred overnight, injuring three people and damaging 14 private buildings, according to local officials on Telegram. A man was injured in another drone attack early on Friday, according to reports. (Reuters could not independently confirm the reports.)