SAN FRANCISCO (Reuters) - Yahoo Inc plans to integrate Twitter into its collection of websites, as the company seeks to enhance the appeal of its online properties with popular social networking features.

The partnership will allow web surfers to view the short, 140-character messages created by Twitter users, dubbed Tweets, directly within Yahoo sites as well as to publish their own Twitter messages without leaving Yahoo.

The move, which Yahoo announced late on Tuesday, comes a couple of months after Yahoo announced a similar deal with Facebook, the world's No.1 social networking site.

Earlier this month, Google Inc unveiled a new service dubbed Google Buzz that replicated many of the social networking features that have made services like Twitter and Facebook Internet success stories.

Facebook and Twitter - which said on Monday that users of its service generate more than 50 million Tweets every day - pose an increasing threat to established Internet giants like Yahoo and Google whose businesses depend on selling online ads to large audiences.

In January, Facebook overtook Yahoo to become the second most visited website in the United States, according to a recent report by web analytics firm Compete. A separate study by comScore showed Yahoo maintaining its No.2 rank with roughly 164 million unique U.S. visitors, while Facebook was the No.4 site with 112 visitors, behind third-ranked Microsoft Corp.

Yahoo said that beginning on Tuesday its Internet search engine results will display up-to-the-second Tweets about various topics, matching the so-called "real time search" capabilities that Google and Microsoft announced in their own respective deals with Twitter last year.

Yahoo also plans to display a live stream of Tweets within other online properties including its email service and sites devoted to sports, entertainment and finance later this year.

Yahoo executives said that the company was looking at ways to make Twitter messages relevant to each property, such as by customizing the selection of messages that appear alongside an article about a particular sporting event, for example.

"We believe that the content and context side of things is very unique," Yahoo Vice President of Communities Jim Stoneham told Reuters in an interview.

Yahoo would not comment on any financial terms involved in the deal with Twitter.

According to some media reports, Microsoft and Google paid a combined $25 million for the right to include Twitter data in their search results.

(Reporting by Alexei Oreskovic; editing by Carol Bishopric)