symbolic directories not recognized by nginx

Hello
I've done several tests : It seems that :
-if I introduce symbolic link to directories inside my web directories, they are not recognized by nginx (err 404)
-if in location tag in the config file I put a symbolic linked directory, it is also not recognized (err 404)
Why is that ?
So how to use symbolic directories with nginx ?
Thank you
I use nginx 1.4.6