A giant outage of Amazon's cloud-computing network in the US, which impacted large users such as media companies, was triggered by an effort to upgrade capacity, the company said Saturday.

The e-commerce giant's Amazon Web Services (AWS) experienced a major glitch on Wednesday that even hit the publishing system of the Washington Post, which is owned by Amazon founder and CEO Jeff Bezos.

"The trigger, though not root cause, for the event was a relatively small addition of capacity," Amazon said in the statement.

The outage impacted many companies, which use AWS services to store data without having to manage their own servers. 

According to US media, the New York subway was among those affected, as well as Roku, a streaming television service. 

The Wall Street Journal said it was hit too. 

AWS, created in 2006, leads the market for remote computing services. Research firms say it has a 30-50 percent market share.