to reflect status of the problem on Friday.

A widespread failure in Amazon.com’s Web services business was still affecting many Internet sites on Friday morning, highlighting the risks involved when companies rely on so-called cloud computing.

The problems, which began early Thursday morning, affected sites including Quora.com, Reddit.com, GroupMe.com and Scvngr.com, which all posted messages to their visitors about the issue. Most of the sites have been inaccessible for hours, and others were only partly operational.

The Web companies use Amazon’s cloud-based service to serve their Web sites, applications and files. Amazon’s customers include start-ups like the social networking site Foursquare but also big companies like Pfizer and Nasdaq.

Amazon, which is a leader in this business, lets these companies rent space on its servers and take advantage of its big data centers and computing power. But that gives the companies little control if the servers fail.

“We don’t think the cloud is enterprise-ready,” said Jimmy Tam, general manager of Peer Software, which provides data backup for businesses. “Are you really going to trust your corporate jewels to these cloud providers?”

Executives at the Web companies that are Amazon customers said that while they knew of the risk, the failure was still frustrating. They said they were investigating options to avoid similar problems in the future.

“Clearly you’re not in control of your data, your information,” said Campbell McKellar, founder of Loosecubes, a Web site for finding temporary workspace that was not available Thursday. “It’s a major business interruption. I’m getting business interruption insurance tomorrow, believe me, and maybe we get a different cloud provider as a backup.”

Other Web companies said cloud computing was a necessity.

“The benefits of the cloud are significant,” said Jeff Janer, chief executive of Springpad, a service that people use to save items online, which went offline as a result of Amazon’s problem. “Amazon as a resource for a company like ours makes an awful lot of sense. We’re just all keeping our fingers crossed that they get back as quickly as possible.”

The problems also affected some functions of the Web site of The New York Times, including the ability to comment on articles.

Amazon did not respond to requests for comment. The company was updating the status of its Web services online and confirmed disruptions at a data center in Northern Virginia. The page said it was unknown when the services would be restored.