If a U.S. citizen becomes engaged to a foreign individual and wants to bring him/her to the U.S. prior to marriage, a K-1 visa must be approved and issued for the foreign fiancé. The application starts in the U.S. with an application that, once approved, is sent to the foreign U.S. consulate where the fiancé resides and will be set for an interview prior to issuance of the visa. Minor children of K-1 fiancés will also be approved for a K-2 visa. Note that the couple will need to provide documentary evidence to support the application that they are indeed engaged to be married and have an ongoing relationship.
Once admitted on a temporary basis with a K-1 visa for fiancés, the couple must marry and file for Adjustment of Status (Green Card) within 90 days of entry.