I'm going to Mongolia and have a 14-hour layover in Seoul with Asiana Airlines from the USA. Do I need a visa to go out of the airport for sightseeing? I'm a Mongolian passport holder.
As stated in Timatic, the database used by airlines:
Visa required, except for nationals of any country with a visa issued by [...] USA [...] if in transit through Korea (Rep.) must: -hold confirmed onward tickets on flights departing within 30 days; when [...] arriving from the country that issued the visa and departing to a third country
So no, you do not need a visa