United Airlines has grounded flights across major US routes due to a technology issue that disrupted operations at several key airports, including Chicago, Denver, Newark, Houston and San Francisco.

The Federal Aviation Administration (FAA) confirmed the nationwide disruption, initially halting all United flights bound for Chicago at their departure points before expanding the advisory.

The airline said the problem had been resolved by late afternoon but warned that delays could continue into the evening.

"Safety is our top priority, and we’ll work with our customers to get them to their destinations," United said in a statement, without providing further details on the nature of the outage.

Travellers across the country experienced confusion and frustration.

On social media, United apologised to affected customers: "Hey there, we apologize for the travel disruption today. Our teams are working to resolve the outage as quickly as possible. Thank you for your patience."

Despite the resolution of the underlying technology problem, travelers were advised to expect ongoing delays and to check their flight status before heading to the airport.