The United Airlines fleet resumed operations mid-morning on Wednesday, after a computer network problem grounded all aircraft for several hours. The problem affected all mainline flights originating in the U.S., including some 235 domestic routes and 138 international destinations. All told, the outage delayed or canceled around 3,500 flights.

“We experienced a network connectivity issue this morning,” United confirmed in a statement. “We are working to resolve this and apologize to our customers for any inconvenience.”

In a subsequent statement issued about an hour later, the company said it was recovering from the problem and restoring regular flight operations.

The U.S. Federal Aviation Administration has lifted the ground stop for United’s regional affiliates.