Microsoft Corporation has upgraded its online mapping service to include three-dimensional tours of 15 American cities, marking another step in its pursuit of Internet search leader Google Incorporated.

With the improvements unveiled Monday, Microsoft is hoping to upstage Google’s popular Earth software, which enables about one-third of the world’s population to obtain an aerial view of their homes and neighborhoods.

Google says it has distributed more than 100 million free copies of the Earth software since June 2005.

Microsoft, the world’s largest software maker, is touting its 3-D version of Virtual Earth as a more compelling alternative that provides photorealistic images of the 15 targeted cities: San Francisco, San Jose, Seattle, Boston, Philadelphia, Los Angeles, Las Vegas, Detroit, Phoenix, Houston, Baltimore, Atlanta, Denver, Dallas, and Fort Worth.

Google so far has been able to brush aside Microsoft’s threats during the past two years to maintain its wide lead in Internet search and generate earnings growth that has mesmerized Wall Street. Google’s stock price has increased by more than fivefold since its August 2004 initial public offering; Microsoft’s shares have inched up by just 6% during the same period.

Executives from Microsoft and Google are among a long list of prominent Internet leaders scheduled to appear at a three-day industry conference called Web 2.0 that begins in San Francisco on Tuesday. Many of the conference presentations are expected to highlight gee-whiz technology like Microsoft’s 3-D maps.

Virtual Earth is designed to work in a Web browser without opening another application — a convenience that Google Earth doesn’t currently offer.

Microsoft is also offering online advertisers a chance to place marketing messages on artificially manufactured billboards dotting the 3-D landscapes. Google also shows ads in its service, but those appear in more mundane thumbnails pointing to a specific location.