SAN FRANCISCO- Microsoft Corp today formally announced it has developed a voice activated phone assistant feature called Cortana, a direct rival to Apple Inc's Siri.

The feature has been rumored for some months and a test version was demonstrated by Joe Belfiore, a Windows Phone executive, at Microsoft's annual developer conference in San Francisco.

The Cortana service, which can take verbal instructions to search the Web, set alarms, make calls and a host of other actions, is still in beta testing, said Belfiore, but will soon be a standard feature on Windows phones.

Belfiore announced that the latest version of Microsoft's smartphone software, called Windows Phone 8.1, will be rolled out to consumers as a downloadable upgrade in the next few months and new phones running the software will be in stores by late April or early May.