Alaska Airlines has revealed that an "unexpected failure" of a critical piece of hardware caused an IT outage that led to its entire fleet being grounded.

U.S. carrier Alaska Airlines resumed operations late on Sunday after briefly grounding all of its flights for about three hours due to an IT outage, marking the second time it has halted its fleet in just over a year.

Alaska Airlines has revealed that its "significant" IT outage on Sunday night was not caused by hackers, but by hardware at one of its data centers failing.