The tracking site Pingdom said the outage began at 11:59 a.m. EDT, with the service returning to intermittent operation around 1 p.m. but then crashing again less than an hour later, CNN reported.
Twitter updated its status blog at 1:42 saying "the issue has been resolved and all services are currently operational" but at 2:16 another update from Twitter said "the issue is on-going."
Twitter said "a cascaded bug in one of our infrastructure components" caused the outage.
In its early years Twitter became notorious for its extensive outages although it has improved considerably.
Thursday's outage was the longest since an hourlong disruption Oct. 7.
"Engineers are currently working to resolve the issue," a Twitter spokeswoman told CNN.