Is there an ISO standard for city identification?

I decided to use IATA codes for cities.

There is an ISO standard for Country code, but none (that I can find) for city code. There is, however; a United Nations Economic Commission for Europe United Nations Code for Trade and Transport Locations system of city identification UN/LOCODE.

Edit: Lined out the wrong name, added the slash to the UN/LOCODE name.

In addition to UNLOCODE, there is also the GeoNames database with its own numeric IDs.

This is probably only better than UNLOCODES if you already need to use the information in the GeoNames database or would prefer an integer-based ID system.