SAN FRANCISCO — In a major expansion, Yahoo Inc. said yesterday that its online search engine index now spans more than 20 billion Web documents and images, nearly double the material scanned by rival Google Inc.

Yahoo's expansion doesn't necessarily mean it produces more useful results than Google, which has long been considered the Internet's most comprehensive database.

But the breakthrough gives the Sunnyvale-based company the bragging rights to a widely watched measurement for assessing the power of an Internet search engine.

Yahoo said its index, boosted by a recent upgrade, covers 20.8 billion online "objects," comprising about 19.2 billion documents and 1.6 billion images.

By comparison, Google said it tracks 11.3 billion objects. That figure consists of the nearly 8.2 billion Web pages that Google proudly touts on its home page and 2.1 billion images, with the remainder of material coming from its group discussions.

Until yesterday, Yahoo hadn't publicly disclosed the size of its search index, but industry estimates had placed the figure somewhere between 6 billion and 8 billion.

"This is a great reason for more people to check us out," said Eckart Walther, Yahoo's vice president of products. "We are more comprehensive than anyone else out there."

In a statement late yesterday, Google spokesman Nate Tyler questioned whether the size of Yahoo's search index had really surpassed its own.

"We welcome innovation in search, but as of this afternoon we have not been able to verify a substantial increase to Yahoo's web index via their search results," Tyler said.

Verifying the index claims of the search engines is virtually impossible because there is no official auditing system, said Danny Sullivan, editor of industry newsletter Search Engine Watch.

Nevertheless, supplanting Mountain View-based Google as the biggest search engine should give Yahoo a potent marketing weapon in a tense duel for industry leadership, predicted Forrester Research analyst Charlene Li.

"The Google brand stands for search and (Yahoo's) strategy has been to undercut that brand," she said.

Yahoo has had its sights set on Google since early last year when it introduced its own search technology and index to end a business partnership between the two companies.