They're the same word!

Both words are same meaning. Kanji of "アメリカ" is "亜米利加". "米国" is short for "亜米利加国" In Japan "アメリカ人" is better. In addition, United Kingdom ： イギリス(英吉利⇒英国)" Italy ： イタリア(伊太利亜) Germany ： ドイツ(独逸)

Both means Amerika jin But it's better to use Kanjis (米国人)

одинаковое значение, без различия.