There is no such thing as an “international copyright” that will protect an author’s work worldwide. Instead, there is a network of countries that have agreed to provide protection to each other’s creators -- to some degree. The level of protection differs from country to country.
Works that are registered with the U.S. Copyright Office are also granted protection in any nation that has a relevant treaty with the United States - that includes over 179 countries such as Canada, China, Russia, India, etc. For a complete list of countries that the U.S. has a treaty with, visit: https://www.copyright.gov/circs/circ38a.pdf
Citizens from most countries can register with the U.S. Copyright Office. Although they don’t need to register before filing a lawsuit, it would be a mistake not to - you’re only eligible to collect $150,000 plus legal fees per wilful infringement if you register.