The US Federal Trade Commission sent letters to 24 internet search companies including Google, Yahoo, and Microsoft Tuesday. The letter contained an update on the FTC's 2002 guidance on search advertising practices.

Particularly, the FTC warned leading internet firms to improve identification of paid ads in search results. This was in response to increasing access of consumers to the internet on new technology, including voice-based online and mobile services.

"In recent years, paid search results have become less distinguishable as advertising, and the FTC is urging the search industry to make sure the distinction is clear," the agency said. Companies who violate the agency's rules against deceptive advertising will be fined.

Google was the no. 1 search engine in the world, accounting for 73.8% of expenditure on search advertising in the US. Yahoo had 6.6% share of online search in the US. Meanwhile, Microsoft was ranked as the no. 3 online search advertising company in the country. The US spent US$17.3 billion on search advertising last year.