What are limits to have with non-muslims friends?

Salam Alaikom, I am living in a european country, and I have some friends here, they have really good values, nice speacking, culture, and they helped me during a critical part of my life. So my question is : are there any limits I should have as a muslim with my non-muslims friends ? Should I consider them to be "friends" or there are islamic rules to call someone "friend" ? Thank you.