Google said Thursday that it had begun testing its radio ad service with a small group of selected advertisers, confirming reports from last month which said the company had begun to hire a sales team and was preparing to launch the effort by the end of the year.

The service would allow the advertiser to select the location, type of station, and the time and day it would like its ads to run. After the ad is played, the Google AdWords system would provide online reports of when and where the ad played.

It was not specified how many stations were participating in the initial launch of the service, although Google referred to it as being in the "hundreds." Markets range from small ones like Tyler, Texas to big areas like New York City and elsewhere.

"The first beta test campaigns recently launched, and you may have already heard these advertiser's ads while driving to work or listening to the radio at home," a member of the Google Audio Ads team wrote Thursday on the Inside AdWords web log.

Ads must be in MP3 format and 30 seconds long. They also must conform to Google's content policies. No date has yet been set for a wider release of the service.

Google Audio Ads was made possible through the search company's January purchase of dMarc Broadbcasting, a company that dealt with the sales, delivery and reporting of radio ads.