Russian President Vladimir Putin has overseen large-scale military exercises with Belarus amid increased US-led NATO activity near Russia's borders.
The Russian president attended the “hot phase” of the drills in the Nizhny Novgorod region, east of Moscow.
The joint military exercise with Belarus — dubbed Zapad — which take place every four years, involve about 200,000 soldiers in total this year, including troops from Armenia, India, Kazakhstan, Kyrgyzstan and Mongolia.
Monday's exercises mainly focused on launching a counter-offensive against enemy forces, with Russia's Northern fleet and air force practicing repelling an attack near its border with Norway.
Putin has said the military drills are not directed against any foreign country, calling them a necessary response to increased NATO activity near Russia's borders and those of its allies.
Hundreds of tanks, aircraft and warships also conduct exercises at several sites across western Russia and in Belarus.
The Russia Defense Ministry released video of warships and tanks opening fire and fighter jets taking off.
According to the ministry, the troops had practiced repelling an aerial attack and that it tested S-400 air defense systems on Crimea.
The exercises included what the Defense Ministry said was the first use in a combat environment of two new robotic fighting vehicles that are equipped with machine guns and grenade launchers.
The joint military drills, which opened last week when Putin met his Belarusian counterpart Alexander Lukashenko, rattled nerves in eastern Europe.
The foreign ministers of Poland, Latvia, Lithuania and Estonia accused Russia of not being transparent about the drills that came amid heightened tensions between the West and Belarus.
Kremlin spokesman Dmitry Peskov, however, brushed off their concerns, saying that “joint military exercises are a regular process.”
Ukraine’s President Volodymyr Zelenskiy also raised concern about such large-scale military drills last week, saying an all-out war with Russia was a possibility.
The Belarusian president observed the military exercise on Sunday near the city of Baranovichi in western Belarus.
He praised Russian and Belarusian troops for their "high state of readiness" to jointly defend their borders and counter "hybrid aggression" from the West.
Lukashenko said he wanted to buy Russia's S-400 air defense systems with a view to placing them on his country's border with Western-backed Ukraine.
“We should get ready — our border with Ukraine is 1,200 kilometers," he said, adding that during talks with Putin they “discussed that we could use the S-400s.”
The US, the EU, Britain, and Canada recently imposed a series of new sanctions on Belarusian entities and officials in a coordinated response to the grounding of an intra-EU flight in Minsk.
Lukashenko has shrugged off the Western pressure with backing from key ally Russia. He has warned that Minsk might be forced to take reciprocal measures that could in turn adversely affect the citizens and businesses of Western countries.