Chicago, July 25 -- United Airlines resumed all its mainline flights after a brief nationwide ground stop triggered by a fire alarm at its Network Operations Centre in suburban Chicago, Fox News reported.

The airline said operations were back to normal late Thursday after employees returned to the primary facility in Arlington Heights, where an emergency alarm had forced a temporary shift to a backup location.

"A fire alarm sounded at our operations centre, which caused employees to move to our nearby backup facility and resulted in a brief nationwide ground stop of United aircraft," United Airlines said in a statement to Fox News.

"Employees have returned to our primary operations centre, and the ground stop has been lifted," the airl...