The airline will only say that it has a "network outage." Its main website, United.com, is down.
United flights to its hub in Newark, New Jersey, are being held at their origin until 6 p.m. Eastern (2200 GMT). Flights to United's San Francisco hub are also being held at their destination.
The airline says it is trying to minimize any impact to its operations.
United Continental Holdings Inc. has been struggling with computer issues ever since it combined some of its major computer systems in March.
Passengers are lighting up social media with complaints about the outages, with some saying boarding passes are being hand-written by United personnel.