United Airlines UAL.O said it had grounded its flights at U.S. airports on Wednesday due to a technology issue, and warned of additional flight delays in the evening.

“Due to a technology issue, we are holding United mainline flights at their departure airports,” the airline said.

United suffered an outage of its weight calculation system, a source familiar with the matter told Reuters.

“The issue affecting United Airlines flights has been resolved and mainline flights are departing once again,” flight tracking website FlightRadar24 said on social media at 9:19 p.m. ET (0119 GMT on Thursday).

The U.S. Federal Aviation Administration said earlier it issued a ground stop for United flights at several U.S. airports.

The ground stops were issued at major United hub airports including Newark, Denver, Houston, Chicago, according to the regulator’s website.

As of 9:25 p.m. ET, flight tracking data from FlightAware showed that 870, or 28 per cent, of United flights had been delayed on Wednesday.

United’s outage came a few weeks after Alaska Airlines grounded all of its flights for about three hours due to an IT outage for the second time in just over a year.

In April 2024, Alaska grounded its entire fleet due to an issue with the system that calculates the weight and balance of its planes.