The popular Web site Twitter is now functioning after a cyber attack took the service offline for several hours.

Twitter said in a status blog earlier Thursday it was "defending against a denial-of-service attack."

Users of the social networking Web site Facebook also encountered problems on Thursday. Unlike Twitter, Facebook never became completely inaccessible, but users did experience long delays while accessing their online profiles.

It is not clear if the attacks on the Web sites are related, but Twitter and Facebook officials say they are working with online search engine Google to investigate.  

Even though both sites are now functioning, Twitter says users will continue to experience longer load times and slow response.

The Twitter outage follows a wave of similar cyber attacks last month that disrupted access to several high-profile U.S. Web sites, including the White House site.

Some information for this report was provided by AFP, AP and Reuters.