The U.S. Copyright Office has this to say:
There is no such thing as an “international copyright” that will automatically protect an author’s writings throughout the entire world. Protection against unauthorized use in a particular country depends, basically, on the national laws of that country. However, most countries do offer protection to foreign works under certain conditions, and these conditions have been greatly simplified by international copyright treaties and conventions. For further information and a list of countries that maintain copyright relations with the United States, request Circular 38a (PDF), International Copyright Relations of the United States.
See also a section on foreign copyrights under Length of Copyright Term.