Ukraine has reacted sharply to Russian President Vladimir Putin's announcement that he has reached an agreement to deploy tactical nuclear weapons in neighboring ally Belarus.

Putin said in an interview with Russian state media on Saturday that a storage facility for tactical nuclear weapons will be constructed in Belarus by July 1.

Ukrainian presidential adviser Mykhailo Podolyak tweeted on Sunday that Putin admits he is "afraid of losing" and "all he can do is scare with tactics."

He also said Putin had stated his involvement in the "crime" of violating the Nuclear Non-Proliferation Treaty.

Ukraine's National Security and Defense Council Secretary Oleksiy Danilov said on Twitter on Sunday that Moscow had taken Belarus as a "nuclear hostage."

He said Moscow's plan maximizes the level of the Belarusian people's rejection of Putin.

A US think tank, the Institute for the Study of War, said on Saturday, "Putin is attempting to exploit Western fears of nuclear escalation."

The institute went on to say Putin has taken this approach "in hopes of diminishing Ukrainian morale and Western aid to diminish the effectiveness of a rumored pending Ukrainian counteroffensive."