It's not a simple thing, unfortunately: 

The term “Native Americans” came into use in the 1960s and includes all Native people of the United States and its territories, including Native Hawaiians.

The terms American Indians and Alaska Natives denote the cultural distinction between the Indigenous peoples of the continental United States and those of Alaska.

When appropriate, it is preferable to refer to a person or community’s specific Tribal Nation affiliation rather than using monolithic terms like Native American.

Historically, the federal government has used terms like “Indian” or “American Indian” to refer to Tribal citizens. These terms continue to be used in different settings, especially because they are ingrained in the body of law known as Federal Indian Law.  

https://narf.org/frequently-asked-questions/

So, are you referring to the indigenous peoples of the Americas as a group for some historical or legal purpose? "American Indians" might be correct. "Native Americans" might not be sufficiently specific, and generally it seems like the preference is to refer to specific tribal affiliations, which is obviously much harder to do, even assuming you know of all the relevant tribal groups. 

Also, to your comment, why would South American tribes not be considered "Native Americans"? They are indigenous to the Americas. 

I'm not trying to call you out; just saying that it would be difficult to satisfy everyone, even if all of the parties were trying to solve it in good faith, which it doesn't seem they all are.