Russia and Belarus signed a deal on Thursday formalising the deployment of Moscow's tactical nuclear weapons on the territory of its ally, although control of the weapons remains in the Kremlin.
Russian President Vladimir Putin announced the deployment of shorter-range weapons in Belarus earlier this year in a move widely seen as a warning to the West as it stepped up military support for Ukraine.
When the weapons would be deployed wasn't announced, but Putin has said the construction of storage facilities in Belarus for them would be completed by July 1.
Also unclear is how many nuclear weapons would be kept in Belarus. The US government believes Russia has about 2,000 tactical nuclear weapons, which include bombs that can be carried by aircraft, warheads for short-range missiles and artillery rounds.
Tactical nuclear weapons are intended to destroy enemy troops and weapons on the battlefield.
They have a relatively short range and a much lower yield than nuclear warheads fitted to long-range strategic missiles that are capable of obliterating whole cities.
The signing of the deal came as Russia braces for Ukraine's much-anticipated counteroffensive. Both Russian and Belarusian officials also framed the step as driven by hostility from the West.
“Deployment of non-strategic nuclear weapons is an effective response to the aggressive policy of countries unfriendly to us,” Belarusian defence minister Viktor Khrenin said in Minsk at a meeting with his Russian counterpart, Sergei Shoigu.
“In the context of an extremely sharp escalation of threats on the western borders of Russia and Belarus, a decision was made to take countermeasures in the military-nuclear sphere,” Shoigu added.