As Russia and Belarus kick off a massive joint military exercise, S-400 missile defense systems have entered combat duty on Belarusian territory.

“The combat crews of the Russian Eastern Military District’s S-400 surface-to-air missile system went on combat duty to ensure air defense as part of the unified regional air defense system of Russia and Belarus during the inspection of the Union State’s troops,” the Russian defense ministry said in a statement, according to Russian state-backed news outlet TASS.

The S-400 deployment is part of “Union Resolve 2022,” a series of joint Russian-Belarusian drills being held from February 10-20 in five training ranges and four airfields spread across Belarus.

“The drills are being conducted to practice the tasks of countering and repelling external aggression in the course of a defensive operation, fighting terrorism and protecting the interests of the Union State,” Russia’s defense ministry said.

Moscow claims the drills do not violate the 2011 Vienna Document, a series of measures aimed at promoting military transparency and confidence-building between participating Organization for Security and Co‑operation in Europe (OSCE) members. Among other provisions, the Vienna Document requires participants conducting exercises exceeding 13,000 troops, three hundred tanks, or two hundred and fifty artillery pieces to notify all other OSCE members in advance and to facilitate certain observation measures.

According to NATO Secretary-General Jens Stoltenberg, 30,000 Russian troops have poured into Belarus in anticipation of the February exercises. The Kremlin has dismissed Western fears of a buildup of Russian forces in Belarus, insisting that the troops deployed there will return home after the conclusion of Union Resolve 2022.

“No one ever said that Russian troops would remain on the territory of Belarus, there was no talk of this,” Kremlin spokesman Dmitry Peskov said. “We are talking about allied exercises, and it is understood that after these exercises are completed, the troops will return to their permanent locations.”

Peskov clarified this week that Russian President Vladimir Putin will not be attending the joint drills in person.

Despite their continued assurances that Union Resolve 2022 will not lead to additional Russian force concentrations in Belarus, Moscow officials say the Kremlin reserves the right to supply Belarus with military hardware.

"I know that [Russian] Defense Minister [Sergey] Shoigu and Belarusian President [Alexander] Lukashenko discussed weapons supplies during their meeting. We talked about modern armaments and that shouldn’t cause any unexpected reaction in the West as there’s nothing extraordinary about that,” said Russian Ambassador to Belarus Boris Gryzlov. "We will supply the types of weapons that we see fit. It’s common practice to reinforce our union state forces and our western border.”

Mark Episkopos is a national security reporter for the National Interest.

Image: Reuters.