i've heard people from south america refer to themselves as 'americans' too. the thing is that US citizens have no other way to refer to themselves than 'american' (unless you include the ridiculous things said in previous comments, which i don't). people from peru are peruvians, etc. but people from the US can only be called americans, so in my opinion that word should refer strictly to them.

but whatever is said here in this comments section isn't going to change anything in the world, so just go on calling people whatever you want and be prepared to make yourself clear in the future when referring to a brazilian as an american. :)