ICBMs are known to be able to carry multiple warheads. The capability to launch against multiple targets in the same time is important when in an all-out war but I can imagine many times that a single strike may be tactically desirable.
So are there ICBMs (land or submarine based) loaded with a single warhead? Can an ICBM "unload" warheads relatively quickly according to tactical needs? Or are single nuclear strikes performed exclusively by air assets (bombers, cruise missiles)?