Russian President Vladimir Putin put the West on notice yesterday during his speech at the St. Petersburg International Economic Forum (SPIEF).
President Putin made clear he knows the West is planning on supplying fighter jets to Ukraine, and that the US insists the F-16's be nuclear capable.
He pointed out that those jets, especially U.S. F-16's, require very large and complicated maintenance, which cannot now be done in Ukraine without fear of being hit with Russian missiles. Thus it would be necessary for the planes to use bases that are presently safe.
Therefore, he said, if those fighter jets takeoff from NATO bases outside Ukraine, and enter Ukraine for battle, Russia will not be able to know if those planes are armed with Tactical nuclear bombs and will have to assume THEY ARE.
Under such circumstances, Russia will have no choice but to hit those NATO Bases, and may have to do so with Tactical nuclear weapons.
Below, video from the SPIEF where Putin talks about this. The video has English sub-titles but is only a small portion of what Putin said. You get the jist of it: