It doesn’t happen very often, but I’ve heard it used that way. It’s usually obvious from context, like I think I heard with “OLED vs. LED”. And as @[email protected] mentioned, it’s used a lot in languages other than English, in my experience in many slavic ones, for example.
I did some research, and apparently, “United States” without “of America” could be a kind of ellipsis. But more likely, it’s just an alternative country name. So I think that makes US an initialism (because you pronounce it as [yu-es]) for an alt (bonus info: this is a final clipping, or apocope, of “alternative”) name.
Linguistics is such a dirt hut…