US mobile subscribers could soon be bombarded with adverts on Yahoo mobile services, after the company revealed it's expanding efforts to cash in on the mobile web.

Later this week, Yahoo will start its first general test of banner ads on its mobile browser-based services. Customers of all the major US mobile operators can access those services, including mobile web-based email, according to the company. The initiative follows smaller, targeted tests, and a general test of search-based mobile advertising in the US and the UK.

Mobile data services have been generally free of advertising, at least in the US, but mobile phones may be an ideal place to grab consumers' attention if the use of data services grows as broad as carriers hope. Relevance, and attractive offers such as coupons, will be the keys to making mobile banners work for both consumers and advertisers, said Greg Sterling, principal analyst at Sterling Market Intelligence.

The size of the ads will vary by type of mobile device, but they won't take up more than one-quarter of any screen, said Yahoo spokesperson Nicole Leverich. Users will be able to click on them and go to either an advertiser's own mobile web page or one that Yahoo has set up for the company. From that page, in many cases the consumers will be able to call the advertiser with one click, she said.

So far, Yahoo is testing banners only on its mobile browser-based services, which include search, news, finance, sports and other offerings as well as browser-based email. In email, ads will appear on the opening page and not on screens where customers write or read email messages, Leverich said. The test doesn't involve the Yahoo Go for Mobile or Yahoo Ready services.

Location-based advertising, which could be a powerful tool for bringing together merchants and customers, isn't part of the test that starts this week. But ads in the test can be targeted at particular services, such as sport or email, and for the time of day and the day of the week, Leverich said.

Mobile advertising isn't likely to take the place of the monthly service charge for data, in Sterling's view. Although free, ad-supported services might attract a bigger audience than paid ones, carriers would be taking a big gamble if they tried to bring in the same amount of revenue from ads, he said.