I certainly would tell them that northern Iraq is currently under US military control. For them to enter a zone of US military control with fighting forces constitutes an attack on US forces.
They certainly wouldn't be flying in there for more than a few minutes, and then once their aircraft are destroyed I would threaten to destroy their ground forces.
This is no different than if Iran were to mount an attack on northern Iraq - same/same.
Geo Strategy Direct is Bill Gertz's web site for his intelligence gathering company. They are probably the most accurate of all the private intel groups, having made many discoveries far ahead of most international or even foreign national intelligence services.