Permanent Representative Vassily Nebenzia penned a letter to the UN Secretary General, outlining Ukraine's plan to detonate a radiological "dirty bomb" so as to create an excuse for NATO forces to enter the fight on the side of Ukraine. We have a portion of that letter, shown below. 

In it, he states "The authorities in Kiev and their Western backers will bear full responsibility for all consequences of such irresponsible action" and goes on to bluntly warn "We will regard the use of the dirty bomb as an act of nuclear terrorism."

It should be pointed out to readers of this story that under Russia's public nuclear doctrine, the Russian Federation makes clear the explosion of a dirty bomb is viewed as the equivalent to a first nuclear strike against Russia; they can and will respond to nuclear terrorism with the use of their own nuclear weapons.