US Americans, US-Americans, US citizens.

"Soviet citizens" was an officially accepted and even encouraged name, though a bit strange considering that "sovyet" simply means "council".

"Citizens of ..." is always the safest form, but rarely needed. Other problems result in the fact that some countries have been renamed by non-democratic regimes, so that exiled citizens use a different name.

More importantly: we should avoid confusing a country with its government whenever we can. Governments can decide for their country, e.g. about declaring war (- regrettable, but formally correct -), but should generally not be trusted to speak on its behalf, even if elected democratically. "The relation between the USA and France" has so many aspects, that adding "... gvts. of ..." may not be too much typing if confusion must be feared.

"Yoosies" and "Youkies" may do when referring to national sports teams, similar to the long-established "Aussies". I would not use either of these names in any other context, although I know that Australians are actually very fond of the abbreviation "Aussies". It may still sound as if they were not being taken seriously, when so addressed by foreigners.