Twitter Inc.'s popular microblogging service was intermittently unavailable to users for significant stretches of time on Thursday, as the company blamed a "bug" that disrupted its system.

Service has now been restored, Twitter said.

Twitter, based in San Francisco, had posted a message reporting the outage on its status blog around 9:35 a.m. PDT. The company updated the post by saying the issue was resolved roughly an hour later, though that message was yet again updated a short time after saying the "issue is ongoing."

Amid the confusion, an individual claiming membership in the "UGNazi" hacker group took responsibility for the Twitter outage, saying it was the result of a so-called denial-of-service attack.

A Twitter spokesman disputed that account, and pointed to a company statement that the "outage is due to a cascaded bug in one of our infrastructure components."