The drills come as Russia steps up its missile attacks on Ukraine, nearly eight months into its invasion and as its troops struggle to make headway on the ground. Putin has recently moved to annex parts of Ukraine, and is mobilizing as many as 300,000 Russian reservists.

The difficulties facing Russia's military in Ukraine have raised concerns that Putin could yet resort to some sort of nuclear strike, although U.S. and European officials say they have detected no change in Moscow's nuclear posture or any unusual movement of its atomic stocks.

The Russian defense ministry said in a statement Thursday that it conducted exercises involving an unspecified number of Yars mobile ground-based missile systems, with troops testing how to hide them from aerial reconnaissance systems and protect them from saboteurs on the ground. More than 3,000 troops and about 300 vehicles took part, according to the statement. It's unclear if those drills will continue into Friday or beyond.