I don't know why you'd care to tell a girl you're attracted to that you lilke her, at all if you are in a relationship already. Don't even pursue or say anything unless you're ready to break up with your girlfriend now. Even just telling the girl you like her and are attracted to her is betraying your current relationship.

As hvert said... It would make me wonder what is not right in your relationship currently to even be tempted by other women like this. Maybe it's time to move on.