SAN FRANCISCO  Google is in discussions to buy Yelp, the Web site that lists and reviews local businesses, according to three people who had been briefed on the negotiations. The talks show how intently the biggest Web companies are setting their sights on the smallest neighborhood businesses, where they see a large and untapped group of advertisers.

Google and Yelp have not signed an agreement, the people said. The people would not disclose the proposed price, but one said it was more than $500 million.

Google and Yelp declined to comment.

“It’s the strongest testament that Google really wants to own this local market, not vicariously but directly,” said Greg Sterling, the founder of Sterling Market Intelligence, a San Francisco company that researches on local businesses and the Internet.

Web companies big and small are trying to give people information about restaurants, news and events within a few blocks or miles of where they are. Yelp dominates the market for local business listings and reviews in big cities.