(Reuters) - A computer problem forced United Airlines <UAL.N> to ground all domestic flights for about an hour on Sunday evening, causing a cascade of delays and annoying customers throughout the United States.

The “ground halt”, which the unit of United Continental Holdings Inc disclosed in a tweet at 8:06 pm ET and lifted about an hour later, follows a series of problems at United and other airlines last year.

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

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

United said it would waive change fees for passengers with a ‘travel waiver’ but passengers took their frustration to social media and the Chicago-based airline started responding to their tweets.

“This has been the worst customer service experience and worst flying service ever experienced in 30+ years,” one passenger tweeted.

“My minor son, the one stuck in Tampa with a tumor in his skull? You just told him he can’t stay in a hotel. What’s your plan?”, another passenger tweeted to which the company responded asking for her son’s travel details.

“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.

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 and Toronto-based Porter Airlines also experienced brief glitches that prompted some flight cancellations.