SAN FRANCISCO  Hoping to earn more from its vast advertising network, Google said it planned to begin selling ads to appear inside videos on sites across the Web.

While the money spent on Web video ads is a small fraction of the $20 billion spent on Internet ads in the United States, Google is hoping to capitalize on both the explosion in online video and the scale of its advertising network, which analysts say includes a vast majority of Web advertisers and hundreds of thousands of Web sites.

The new program, called AdSense for Video, could help Web publishers in that network make more money off their video clips.

While AdSense for Video is the latest in a string of new advertising initiatives that Google has started over the last two years, few, if any, of those initiatives have created substantial revenue for Google. They include Google’s programs to place ads in newspapers, on radio and on TV stations and on a variety of new formats on the Web and on cellphones.