Are there any (international/non-iranian) websites that allows Iranians to freelance?
I tried a few but they don't let me register due to US sanctions.
There are freelancing sites like Upwork, Fiverr, Freelancer, PeoplePerHour on which registration is not specified to any particular country. On these sites, you can freelance from anywhere around the globe. All you need is good skills that are needed by the buyers/clients and services that can attract clients.
However, in your case, as Iran comes under US sanction, you get left with very few sites. One of them is 99designs.com, and it allows people from Iran too.