[orwin]

Just call them USians. When I say 'americans' I often talk about north Americans, I use USians to disambiguate.

[sadeshmukh]

I have never heard American refer to anything but a United States resident, so I seriously doubt you. USian is disrespectful to the preferred demonym, if you do care about that.

[api_or_ipa]

Pedantically, the formal name of Mexico is Estados Unidos Mexicanos, or United States (of) Mexico. USians would then be ambiguous.