International flights were not affected, the Federal Aviation Administration said. The number of flights that were affected was not known.

“The ground stop has been lifted. We’re working to get flights on their way,” the company said in a tweet.

“We are working as quickly as possible to resolve this issue and get out customers to their final destinations,” a company spokeswoman Maddie King said in an emailed statement earlier.

United was just one of several U.S. airlines to experience disruptions due to glitches last year, an issue that prompted some to blame Texas-based Sabre (SABR), a software company.

For more on United Airlines, watch Fortune’s video:

In October, thousands of United passengers were delayed worldwide after a computer glitch temporarily halted departures.

In June, software needed to dispatch United’s flight plan briefly lost functionality and in July, the same airline’s flights were disrupted after a computer problem blocked access to reservations records.

Last week, Air Canada (AIR-CANADA-INC) and Toronto-based Porter Airlines also experienced brief glitches that prompted some flight cancellations.