Russia and Belarus inked an agreement on Thursday to formalize the deployment of Russian tactical nuclear missiles on Belarusian territory, citing increased tensions with the West as the impetus for the move.

“In the context of an extremely sharp escalation of threats on Russia’s and Belarus’ western borders, a decision was made to take military-nuclear countermeasures,” Russian Defense Minister Sergei Shoigu was reported as saying by the TASS news agency.

The deployment of the missiles was first announced by President Vladimir Putin in March. Since invading Ukraine last year, Putin has said repeatedly that Russia would be ready to use nuclear weapons if needed to defend its “territorial integrity”.

NATO said at the time it did not see any need to adjust its own nuclear posture, though it said Putin’s nuclear rhetoric was “dangerous and irresponsible“. Ukraine said Russia’s ally Belarus had been “taken hostage” by Moscow.

Moscow will retain control over the weapons and any decisions on their use, Shoigu said.

TASS quoted him as saying that Iskander-M missiles, which can carry conventional or nuclear warheads, had been handed to the Belarusian armed forces, and some Su-25 aircraft had been converted for the possible use of nuclear weapons.

“Belarusian servicemen have received the necessary training in Russian training centres,” Shoigu was quoted as saying.

He added that the agreements signed with his Belarusian counterpart covered the procedure for establishing a “special storage facility for nuclear weapons on Belarusian territory.”

Tactical nuclear weapons refer to lower-yield weapons designed for battlefield use, as opposed to strategic ones capable of wiping out entire cities. Russia has not disclosed how many tactical nuclear weapons it has.