United Airlines flights were resuming as scheduled at Newark Airport after a power outage at the airline's control tower there severely backed up outgoing flights, officials say.

The FAA says that some sort of equipment problem or outage caused delays of over two hours at Newark Airport Monday evening. Six United Express flights were also canceled, according to the airline.

The Port Authority says the United Airlines control tower at Terminal C experienced a power outage shortly after 5 p.m. Power was restored about 45 minutes later, but it went out again shortly afterward.

Power was again restored shortly after 7 p.m., and has since been running, and flights have resumed, the Port Authority says. There were no residual delays, according to United.

It's not clear what caused the outage. A source says a backup generator didn't kick in like it was supposed to -- and that meant planes couldn't push back from the gate nor could arriving flights come in.

United Airlines has a hub at Newark Airport and makes up a large percentage of the airport operations there.

One passenger tweeted a photo from inside a plane held on the runway, saying, "This has been my view for 2 hours as we're stranded at Newark Airport due to power outage."

He added, "We just had to turn off the engine to refuel since we've idled for so long."

United Airlines responded to him in a tweet, saying the flight was delayed "due to air traffic control conditions impacting our flight operations."

Barbara Harrison of Westwood was arriving on a flight from Aruba and had to wait on the tarmac for about an hour.

"We had to wait, they said there was a blackout, so we had to change our gate," she said. "Everybody was patient and we got home safe."

Another passenger complained: "Radios are down and they are trying to manage comms with walkie-talkies and cell phones. Huge delays."