My dictionary [3] has many definitions for citizen, none of which are very specific about being ascribed to a country, although the example does use a country:
> 1 : an inhabitant of a city or town especially : one entitled to the rights and privileges of a freeman
> 2a : a member of a state
> b : a native or naturalized person who owes allegiance to a government and is entitled to protection from it
> // She was an American citizen but lived most of her life abroad.
> 3 : a civilian as distinguished from a specialized servant of the state
> // Soldiers were sent to protect the citizens.
[1] https://economictimes.indiatimes.com/nri/visa-and-immigratio...
[2] http://blogs.reuters.com/india/2009/10/02/why-is-china-issui...
The term state in that context means country, which is why citizen is a synonym for national. in the Oxford English Dictionary, that is the first definition, and secondarily (and more historically than anything) it refers to citizens of a town. It might be in use locally, but I have never heard anyone use the term citizen in that way other than poetically, and I have spent time in a number of English speaking regions.