Amazon’s cloud-computing service, Amazon Web Services, is experiencing an outage in its eastern U.S. region, causing unprecedented and widespread problems for thousands of websites and apps.

Amazon is the largest provider of cloud computing services in the U.S. Beginning around midday on the East Coast, one region of its “S3” service based in Virginia began to experience what Amazon, on its service site , called “increased error rates.”

In a statement, Amazon said as of 4 p.m. E.T. it was still experiencing “high error rates” that were “impacting various AWS services.”

“We are working hard at repairing S3, believe we understand root cause, and are working on implementing what we believe will remediate the issue,” the company said.

WHY S3 MATTERS

Amazon’s Simple Storage Service, or S3, stores files and data for companies on remote servers. It’s used for everything from building websites and apps to storing images, customer data and customer transactions.

“Anything you can think about storing in the most cost-effective way possible,” is how Rich Mogull, CEO of data security firm Securosis, puts it.

Since Amazon hasn’t said exactly what is happening yet, it’s hard to know just how serious the outage is. “We do know it’s bad,” Mogull said. “We just don’t know how bad.”

The problem affected both “front-end” operations – meaning the websites and apps that users see – and back-end data processing that takes place out of sight. Some smaller online services, such as Trello, Scribd and IFTTT, appeared to be down for a while, although all have since recovered.

The corporate message service Slack, by contrast, stayed up, although it reported “degraded service ” for some features. Users reported that file sharing in particular appeared to freeze up.

The Associated Press’ own photos, webfeeds and other online services were also affected.

TECHNICAL KNOCKOUTAGE

Major cloud-computing outages happen periodically. In 2015, Amazon’s DynamoDB service, a cloud-based database, had problems that affected companies like Netflix and Medium. But usually providers have workarounds that can get things working again quickly.

“What’s really surprising to me is that there’s no fallback – usually there is some sort of backup plan to move data over, and it will be made available within a few minutes,” said Patrick Moorhead, analyst at Moor Insights & Strategy.

(AP)