Russian military forces and hardware began arriving in Belarus this week for the "Allied Resolve" drills to be held near the former Soviet republic's western border with NATO members Poland and Lithuania, and close to its southern flank with Ukraine.
Western states fear that Russia, which has massed tens of thousands of troops on its borders with Ukraine, is planning a new assault on a country it invaded in 2014. Russia has denied such plans.
Lithuania has also said the build-up of Russian troops there was a threat to its security.
Russia's defense ministry said two S-400 battalions - which typically include eight anti-aircraft missile systems each - have started moving to Belarus from Russia's Far East by train, Interfax said on Friday (January 21).
Moscow has said that 12 Sukhoi Su-35 fighter jets and a Pantsir missile system would also be deployed to Belarus for the drills.