Can you declare war on a declared friend in Civ 6?

In Civilization 6, are you allowed to make a declaration of war against someone that you have a formal friendship relationship with?

Yes, you can declare war on friends. Of course this will be met with diplomatic penalties such as warmongering and denouncing, but nonetheless you can do it.

EDIT: As tubes pointed out, you cannot declare war on Allies.

I don't think the accepted answer is true anymore. I want to attack a civilization (a declared friend, not an ally) which is attacking an allied city state, but I am unable to declare a surprise war:

Could this have been changed in a later patch?

Whether or not it is possible to declare war using a causus belli I do not know.