The word United States can be used as a noun or an adjective within sentences.
What AP Style US Suggests?
1. If you are using ‘United States’ as a noun, you should capitalize it by giving a space between ‘United’ and ‘States’.
• There are a few top universities in the United States.
2. You can use U.S. (as an adjective) without any spaces between both letters. For example,
• A U.S. citizen asked me to join his local company.
Note: According to AP Style, you should use US (with no periods and space) in headlines. For example,
• US postal employee charged… (Firstcoastnews.com)