Yahoo on Wednesday rolled out a test of keyword-based advertisements that will be displayed on mobile phones and other portable devices when users make search queries. The trial will cover the United States and the UK, expanding a previous effort in Japan.

Although the market for ads on mobile phones is still relatively tiny, companies are hoping it will eventually prove just as lucrative as traditional Web based ads. Microsoft purchased mobile advertising company Third Screen in May for this reason. Yahoo is currently working with about 100 advertisers for the test, who pay each time a user clicks a link.