Thousands of US passengers were facing delays after United Airlines halted many departures in the wake of a systemwide problem.

"Due to a technology issue, we are holding United mainline flights at their departure airports," the carrier was reported as saying on Wednesday, referring to major routes.

"We expect additional flight delays this evening as we work through this issue. Safety is our top priority, and we'll work with our customers to get them to their destinations."

The Federal Aviation Authority, which regulates flying in the United States, issued ground stops at several major airports.

The orders affecting airports in Denver, Newark, Houston and Chicago only apply to United, it said on its website.

The outage is the latest problem to afflict America's aviation sector.

Last month, Alaska Airlines suffered an IT problem that left its planes on the ground for several hours.

That came after air traffic control systems went down at a Newark area airport on more than one occasion this year, shaking passengers' faith in the system.

In January, a mid-air collision near Washington's Reagan National Airport involving a passenger jet and a military helicopter claimed dozens of lives. (AFP)