How to say it accurately?

I have a friend, and she is a nice girl. We are friends with her for a long time already, I thought that our relations are no more than friendship. But recently it was clear to me that she is feeling sense to me more than friendship, even very much, she loves me. I concern to her very well, but I can't tell that I can fell in love with her. I try to explain it to her, but how to make on more accurately that she would not take offence?