If cities could talk, that would be correct.
It could maybe be a better example if you would use the plural "Sono delle cittadine americane". This sentence could both mean "they are american citizens" or "they are American small cities". The context will help you to understand what the Italian sentence means, based on if you are talking about people or cities.
- Ti presento Rita, Linda e Melissa. Sono delle cittadine americane. Sono qui in vacanza.
- Oggi parleremo di Atlantic City, San Diego e Tampa che, come saprete, sono delle cittadine americane.
Every English speaker uses the term American to refer to people/ things from the US. There is no other adjective for that nationality.
It's just a matter of context: 'European' refers to anything from the continent of Europe, but 'European citizen' means someone from a member state of the European Union. If there was a huge country called the United States of Africa we would probably use the term 'African' in the same way.
What about 'us citizen'? At least that's what I use... I'm just a stickler for precision...with america being two continents and the us being just a little part of them...just imagine: 'I'm from America' 'North or south?' 'North' 'Alors êtes vous de Quebec??' 'No I'm not from latin America'...and so on. Truth to be told almost all Americans (also the ones outside the us) use american as a synonym for us citizen... For a mexican it doesn't sound wierd to cross the border from Mexico to america.