The Internet media company said it will start an initial public evaluation later this week in the United States of slimmed-down banner ads and similar advertising to the small, but growing number of users of Yahoo's Mobile Web service.

"The challenge is that because cell phone screens have limited real estate, ads have to more relevant," said Julie Ask, an analyst with JupiterResearch. "There is an extra burden" compared with ads delivered to computer users on the Web.

The brand advertising trial comes a month after Yahoo began a parallel test to deliver relevant text advertisements tied to searches phone users can perform on mobile Web browsers. That test is taking place in the United States and Britain.

Banners ads are nothing new on cell phones. Start-ups like Third Screen Media of Boston already work with ad agencies, buyers and mobile carriers to deliver ads to wireless users.

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