United Airlines says it's "in the process of resuming normal operations" after a network outage that caused problems at airports and affected its website Tuesday afternoon.
The outage began at about 3 p.m. EST, causing delays and cancellations. United.com was also down.
United flights to its hub in Newark, N.J., were held at their origin until 6 p.m. Eastern. Flights to United's San Francisco hub were also held at their destination.
The airline says it is trying to minimize any impact to its operations.
The company said in a statement, "United apologizes for the disruption caused to travelers at affected airports and is reaccommodating customers as quickly as possible."
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 social media with complaints about the outages, with some saying boarding passes are being hand-written by United personnel.
According to reports, there were long lines and significant delays for United passengers trying to board flights at Newark Airport early Tuesday evening.
Daniel Dobrott told NBC 4 New York in a tweet he was "stuck on a plane" that had just landed at Newark from Los Angeles.
"No notice from pilot as to why," he said. "Was finally told after 30 minute-hold on runway we can proceed to gate. Many people furious about possible missed connections."
Another passenger in Houston waiting to take off for Newark said he'd been on the ground for nearly three hours with no news or updates.
"We're hoping to get there soon," Eric M. Ogden tweeted to NBC 4 New York. "Still on the plane at the gate with the door closed, with no news."
In a statement, United said it is issuing a waiver policy permitting customers on affected flights to cancel or rebook their itineraries without penalty.