The Federal Trade Commission said the firms have not been complying with guidelines put in place in 2002 requiring them to distinguish clearly between search results and ads -- including paid search results, displayed as a result of search queries -- Digital Trends reported Wednesday.
The FTC sent letters to general-purpose search engines AOL, Ask.com, Bing, Blekko, DuckDuckGo, Google, and Yahoo as well as a number of search engines specializing in shopping, travel and local businesses.
Updating the guidelines for the first time in 11 years, the FTC said the requirements focused on "the need for visual cues, labels, or other techniques to effectively distinguish advertisements, in order to avoid misleading consumers."
Background shading formerly used by many search engines on their web pages to distinguish ads from genuine search results had become "significantly less visible" in recent years, the FTC said.
In addition to visual cues like shading, the FTC said, search engine companies must use "large and visible" text labels to indicate advertising content on their web pages.