(CNN) United Airlines resumed operations Sunday night after a computer problem temporarily grounded all domestic mainline flights, two sources familiar with the incident told CNN.

"The ground stop has been lifted," the airline announced on Twitter. "We're working to get flights on their way. We apologize for the inconvenience to our customers."

UPDATE: The ground stop has been lifted. We're working to get flights on their way. We apologize for the inconvenience to our customers. — United (@united) January 23, 2017

Earlier, United spokeswoman Maddie King told CNN that the flights were grounded due to an IT issue.

"We have issued a ground stop for all domestic mainline flights due to an IT issue," King said. "We are working as quickly as possible to resolve this issue and get our customers to their final destinations. We apologize for the inconvenience to our customers."

Mainline flights are those operated by an airline's main operating unit, not regional alliances or subsidiaries.

Read More