President Vladimir Putin says Russia has reserved the right to use all options in Ukraine, but that Moscow would use force only as a last resort.
Thousands of houses were also flooded after torrential rains, storms and landslides hit Russia’s Krasnodar region near the Black Sea.
Russian troops dug a 8km long canal to keep fires caused by a record heatwave away from a nuclear arms site, local media said on Saturday.
Russia was preparing to turn off gas deliveries to neighbouring Ukraine on Thursday, raising the spectre of disruptions to European Union supplies.