Ms. Engle cited a 2012 study by SEOBook, a search strategies company, which found that “nearly half of searchers did not recognize top ads as distinct from natural search results.” Top ads are advertisements that appear immediately above the list of search results.

Image A screen shot of a recent Bing search.

Those ads are often set apart by background shading, to distinguish them from other search results. But the F.T.C. said that the shading was often too light and failed to differentiate the ads from nonpaid material. In addition, it warned, the formats used in one type of device — say, desktop browser pages — often did not work for different devices, like mobile smartphones.

Specialized search results — for example, from a search engine that focuses strictly on one industry, like airlines or hotels — are sometimes based at least in part on payments from a third party, the F.T.C. said. “If that is the case, it is also a form of advertising and should be identified as such to consumers,” the agency said.

Image A screen shot of a recent Yahoo search.

The issue came up during last year’s F.T.C. investigation of Google, when other companies accused Google of displaying shopping results based not on what would best suit a customer, but rather on the potential return to Google.