Typically only US citizens are called Americans.
This is a sore point for many citizens of all the nations from Mexico on south... From their perspective, 'America' means the continents of North America, Central America and South America, and is *definitely* more than the USA. For a USA citizen to call himself an 'American' is to either dismiss the rest of the hemisphere or claim it all for the USA, either of which are annoying. If you tell somebody in Mexico that you are an 'American' (when you mean from the USA) you are likely to offend them.
That said, I think that Europeans (and probably many others) freely say 'Americans' when they mean 'USA-ans'. There actually isn't any convenient word for 'United States-ian'.