Seattle: Amazon.com's cloud computing unit said that the outage that shook up a sizeable part of the internet Tuesday was caused by human error.

The Amazon Web Services division said in a post-mortem published on its website Thursday that its team was working to fix a problem that slowed down the billing system for S3, a widely-used AWS service.

The AWS outage lasted slightly more than four hours.

Through S3, companies and individuals can store their data on Amazon's server farms. S3 also houses the data that underpins a wide array of other AWS services, including some computing processing functions. It works as a basic building block of Amazon's cloud, which in turn is a major pillar of the modern internet.

To fix the slow-down issue, engineers in AWS' Northern Virginia operation - one of the largest cluster of data centres run by the company - needed to take down a small number of servers.