Google joined a handful of other companies by announcing that it is delivering mobile banner ads.

Companies that use Google's AdWords to advertise online now have the option of displaying a banner ad on cell phones instead of simple text. Google includes a text line under each banner identifying it as an advertisement. Clicking on the ad opens a mobile Web page for the advertisement.

A number of other companies already display banner ads on mobile Web pages. Yahoo, AdMob and Third Screen, which is now owned by AOL, are among companies that display banner ads for advertisers. Microsoft displays banner ads on some of its mobile Web pages, such as MSNBC and other MSN mobile Web pages.

While many other companies are already supporting mobile banner ads, mobile Internet usage is still relatively low, so Google probably hasn't missed an opportunity, said Greg Sterling, an analyst following mobile search and advertising as part of a joint venture between Sterling Market Research and Opus Research. "It's not like consumer behavior is established and they're late to the game," he said.

In addition, advertisers are still mostly experimenting with mobile advertising, so few are wedded to any particular ad network, he said.

Google could actually have an advantage over competitors in the mobile banner-ad market because of the way the search giant charges advertisers. All mobile banner ads from Google are priced on a per-click basis. Other advertising platforms often charge based on impressions, or how many times someone views the page with the advertisement, and in mobile, those rates are often quite high, Sterling said.

With Google, advertisers can test out mobile advertising with less risk because they only pay for what users click on, he said. That could attract some advertisers who are uncertain about the effectiveness of mobile advertising.

Google and other Internet companies are increasingly interested in targeting mobile users because they believe the mobile market represents a new opportunity for earning advertising revenue. So far, no company has emerged as a clear leader in mobile advertising.