Wal-Mart Stores Inc., the largest US retailer, launched a free Internet classified ad service on Monday in an alliance with Oodle.com, a California start-up that is seeking to challenge the dominance of Craigslist and eBay-owned Kijiji.

The classifieds listings went up quietly last week on Wal-mart's site, making it the first retailer to attempt to enter territory that was traditionally dominated by local US newspapers, which have been steadily losing ground during the past decade to online competitors.

Craigslist currently has 30.3 million unique users monthly, according to ComScore Media Metrix, while Oodle claims 5 million.

This is good for Wal-Mart because it provides a different experience for consumers, and it benefits Oodle because it expends the reach of our network, Oodle Chief Executive Craig Donato said in a statement.

Oodle.com was launched three years ago in the San Francisco area by former eBay Inc. and Excite executives.