Moscow has condemned US military proposals to develop new, smaller atomic bombs mainly to deter any Russian use of nuclear weapons.
Russia's foreign minister called the move "confrontational", and expressed "deep disappointment".
The proposals stem from concerns that Russia may see current US nuclear weapons as too big to be used.
This could mean, according to the US military, that those weapons are no longer an effective deterrent.
Russia's counterblast
The Russian foreign ministry accuses the US of warmongering in its statement, issued less than 24 hours after the US proposals were published.