US or United States for the place, American for the demonym. US Citizen is far too clunky to use casually and I think it's very unlikely that people would misunderstand given that people from other countries in the Americas tend to use a demonym associated with their country rather than with their continent. This is especially true among North Americans and people from South America are far more likely to call themselves South Americans than Americans.
Calling the country "America" is a little awkward to me though. In that case, US or United States is no more clunky and considerably more specific. Even then, it doesn't bother me particularly when people say "America" to mean the US since it's become conventional enough at this point that there are very few misunderstandings.