Yahoo is to begin delivering graphical ads to mobile phones as part of a test of how it can extend corporate brand marketing on the web into the wireless market, the company has said.

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 cellphone screens have limited real estate, ads have to more relevant," said Julie Ask, an analyst with JupiterResearch.

"There is an extra burden" compared to 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 cellphones. Start-ups like Third Screen Media of Boston already work with ad agencies, buyers and mobile carriers to deliver ads to wireless users.

But as more and more US mobile phone users own phones with fast internet access, major internet players are eying the market. On the computer web, Yahoo is the largest provider of image-based, brand advertising and the No. 2 supplier of online search advertising behind rival Google.

Because consumers carry mobile phones with them around town, advertisers are willing to pay several times more for wireless ads as they for standard computer ads over the Internet.

"The bottom line is there should be a premium for mobile advertising," Ask said.

Now consumers can click on interactive ads to learn more details about an advertiser's offer or to call the advertiser. The ads, which measure 150 pixels by 21 elements, are images that take up a small portion at the top of the screen.

Yahoo's Mobile Web service is available on most phones offered by major wireless operators in the United States. It gives consumers access to a basic set of Yahoo services, including search, e-mail, news, stock quotes and sports.

JupiterResearch estimates that 10 per cent of US mobile users browse the internet on their phones. But only 2-3 per cent check the web on their phones at least five times a week - a key measure of active Internet use and advertising readiness.

"Now is the time for Yahoo to experiment," Ask said.

As more US users sign up for high-speed mobile internet services and buy newer phones, "Eventually consumers will treat the phone as a mini-web experience," Ask predicts.