Parts of Twitter became inaccessible a day before thousands of fans are expected to start tweeting about the Olympic Games.
The Twitter.com site was unreachable for almost an hour on Thursday.
The service was still accessible via its mobile site and through third-party applications.
In a message to users, the company said: "Howdy folks, looks like we're experiencing a small interruption of Twitter.com and some mobile clients."
On its network status page, the company wrote: "Our engineers are currently working to resolve the issue."
It is not yet known what has caused the fault.
Twitter is expected to be one of the main social networks to be used by both fans and athletes during the Games.
Sporting events regularly attract the service's biggest audience.
During Spain's Euro 2012 win, there were over 15,000 tweets every second.