Amazon.com had more trouble with its Web site Monday, just days after unspecified system issues knocked the online retailer offline for more than two hours.
The extent and duration of the latest outage isn't yet known, though the site seemed to be functioning by mid-afternoon, based on tests by The Associated Press from three U.S. locations. Amazon officials did not immediately return a phone call Monday.
On Friday, Amazon's site shut down for more than two hours during the business day, giving an error code to anyone visiting it.
"Amazon's systems are very complex and on rare occasions, despite our best efforts, they may experience problems," the company said in a statement explaining Friday's outage.
A similar "service unavailable" message greeted some visitors Monday. The generic message was later replaced by an apology and a promise to restore service quickly.
Keynote Systems, a California-based company that measures Web site performance, would only say it was monitoring the "fluid situation" at Amazon.
Outages at Amazon are rare, though the retailer had brief disruptions in 2006 because of a Thanksgiving Day sale on Microsoft Xbox 360 video game machines.
Earlier this year, Amazon had trouble with a separate service, Amazon Web Services, which offers other companies pay-as-you-go data storage. Several companies temporarily lost access to their own files when that system went down.