SAN FRANCISCO (AFP) - Amazon has filed suit seeking to crack down on more than 1,000 people offering fake reviews to sellers on its online marketplace.

The US online retailer said its investigation showed a flourishing marketplace for "five star" reviews of products and services - often for a price of US$5 (S6.93) through the online service platform Fiverr.

"At a time unknown to Amazon, each of the defendants enrolled at Fiverr and began offering to write Amazon reviews for US$5 or more," said the lawsuit filed in Amazon's home state of Washington last week.

"Amazon has conducted an extensive investigation of the defendants' activities on Fiverr, including purchasing 'reviews' for products and communicating directly with some of the defendants."

In many cases, "the reviews posted are entirely fake, and the 'reviewer' has no knowledge whatsoever of the product or book being reviewed," Amazon's complaint said.

"Most defendants promise glowing reviews of products or books they have never seen." Amazon's complaint said the fake reviews are undermining the system it pioneered of online rating by purchasers.

"Unfortunately, a very small minority of sellers and manufacturers sometimes tries to gain unfair competitive advantages for their products on Amazon.com," the lawsuit said.

"One such method is creating false, misleading, and inauthentic customer reviews. While small in number, these reviews can significantly undermine the trust that consumers and the vast majority of sellers and manufacturers place in Amazon, which in turn tarnishes Amazon's brand."