Yahoo Inc. suffered availability problems on Friday that affected its home page as well as other of its Web sites and services for a sustained period of time.

Yahoo, which has some of the most popular sites and online services worldwide, first experienced problems on its home page at around 5:50 a.m. U.S. Pacific Time, said Dan Berkowitz, senior communications director at Keynote Systems Inc. , a global provider of mobile and Internet test and measurement services.

Yahoo.com's operations began getting back to normal at around 7:15 a.m., said Berkowitz. At its worst point, Yahoo.com's availability dropped to around 60 percent, meaning that 4 out of 10 visitors couldn't access the page, he said, citing data collected from Keynote's global network of monitoring computers, which check the availability of the world's most popular sites.

NetCraft Ltd. also detected the problems , saying in a short note that the intensity of the performance issues varied among geographical areas and also among Yahoo services.

"The Yahoo.com home page was inaccessible for several hours from our London monitoring station and responded more slowly than usual from several locations in the U.S. Yahoo Search appears to have experienced lengthier availability problems than the home page and other Yahoo services," NetCraft reported.

A variety of bloggers also reported trouble Friday morning accessing other Yahoo services like Yahoo Messenger and Yahoo Mail, as well as other Yahoo sites like the Flickr photo sharing site and the news aggregation site Yahoo News.

Ideally, Yahoo and other major Internet players would never face Web site outages, but they all do, said industry analyst Greg Sterling from Sterling Market Intelligence. The key is to make sure that significant availability problems happen infrequently. Otherwise, users and advertisers get worried and upset, he said.

Yahoo's problems on Friday probably affected fewer users than usual because many people are taking time off in the U.S. and are away from their computers due to the July 4th Independence Day holiday, Berkowitz said.

Yahoo acknowledged the problem and blamed it on "network carrier-related issues," a Yahoo spokeswoman said via e-mail.

"For a brief period early this morning, some Yahoo users may have experienced delays in accessing Yahoo services in the U.S.," she wrote. "The problem has since been identified and corrected by our network vendor."