Russia and Belarus will begin 10 days of joint military drills on Thursday, setting in train one of the most overtly threatening elements of the Kremlin’s buildup of forces around Ukraine’s borders.

Valery Gerasimov, the head of the Russian general staff, arrived in Belarus on Wednesday to oversee the drills.

Russia has moved up to 30,000 troops, two battalions of S-400 surface-to-air missile systems and numerous fighter jets into Belarus for joint training exercises with the Belarusian army. Satellite imagery shows much of the hardware has been moved to locations close to the border with Ukraine.