Starting in January 2015 Amazon offered authors and publishers the opportunity to buy ads on its site. The feature was only open to authors with titles in KDP Select, but earlier this year Amazon made it available to all KDP users.
Self-Publisher-Bibel.de has noticed that Amazon has updated its KDP support pages with new instructions on how authors can advertise their ebooks on Amazon.com. The ebooks have to be available in KDP, they must be written in English, and the ebooks also have to meet Kindle Author Creative Acceptance Policies(PDF).
So long as the ebooks meet the minimum requirement, authors can buy ads on other books listing pages, on the Kindle and Fire tabletes, and they also have the option to buy ads which infest Amazon's search results.
This service is apparently only available on Amazon.com at this time. Or rather, we have a report that it is not available through Amazon.de, and I could not find it on Amazon.co.uk.
But since there is no mention that this service is restricted to US authors, all authors should be able access it through their KDP.Amazon.com account.