Power outages at a United Airlines control tower at Newark Liberty International Airport delayed flights on Monday evening for about two hours.

The Port Authority of New York and New Jersey says United's control tower at Terminal C suffered a power failure around 5 p.m. The power was restored briefly but failed again. It was restored again around 7 p.m.

Flights have resumed. Chicago-based United Airlines says there are no residual delays.

Passengers have tweeted complaints about being stranded at the airport and idling for hours.

It's unclear what caused the power outages at Newark, where United has a hub.

United's parent company is United Continental Holdings Inc.