A fire apparently set by an employee at a Chicago-area air traffic control center led to the cancellation of more than 1,700 flights at the city's two major airports, snarling air traffic across the US, officials said.

Flights resumed at O'Hare International Airport, one of the world's busiest, after a delay of about four hours, but there were bottlenecks across the entire air system that carriers expect to last through the weekend.

The blaze at the facility in Aurora, near Chicago, appeared to be set by a man who suffered self-inflicted knife wounds, Aurora police said. There were no indications that it was an act of terrorism and the blaze was quickly extinguished, they said.

The fire was set by a longtime employee of Harris Corp , which provides equipment and technical support for the Federal Aviation Administration facility in Chicago and many others, U.S. government officials said. The name of the employee has not been released.

The FAA is still assessing the damage, which may be significant, but the agency hopes to restore air traffic to relatively normal levels over the next few days, the officials said.

Air traffic was being handled by other control centers in the region, including Minneapolis, Indianapolis and Cleveland, officials said.

The suspect was identified as a 36-year-old man who was apprehended in a minor struggle and is expected to survive his injuries, law enforcement officials said at a news conference adding the fire was an isolated incident.

"We don't know what the motive was at this time," said Tom Ahern, a spokesman for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives. No criminal charges have yet been made.

The incident caused major delays at O'Hare and the domestic hub Midway International Airport, affecting flights from almost every state and routes with Europe, Asia and Latin America.

Local broadcaster WGN quoted a federal law enforcement official as saying the suspect was a disgruntled employee who tried to sabotage the center, which is crucial for air traffic.

The man entered the building's basement and cut wires for air traffic control equipment and also doused the wires with gasoline and set them on fire, WGN reported.

There were 1,754 flights into and out of the two airports that were canceled by 5 p.m. CDT (2200 GMT), according to tracking website flightaware.com, leaving thousands of travelers stranded.

"There's cascading delays because nothing can take off bound for Chicago from anywhere," said Doug Church, spokesman for the National Air Traffic Controllers Association, a union of air traffic controllers. "The impact is national and major."

DELAYS THROUGH THE DAY

At O'Hare, passengers were scrambling to find alternative transportation or bracing for long delays.

"I'm shocked at how calm everyone is. With everything going on in the world, maybe we're all managing our expectations. It's a fire in Aurora, it's not ISIS," said Cynthia Stemler of the Chicago suburb of Lake Bluff, who was heading to Newark, New Jersey, in a reference to the militant Islamic group at war in Syria and Iraq.

O'Hare is the main hub for United Airlines and a major hub for American Airlines. The airport averaged about 2,700 flights a day in August with a daily average of about 220,000 passengers passing through in the month, according data posted on its website.

Southwest Airlines Co suspended all flights through the day at Midway and Milwaukee's General Mitchell International Airport, the airline said in a statement.

The FAA said employees were evacuated from its control center in Aurora when the fire broke out, which government officials said occurred at about 5:30 a.m. CDT (1030 GMT).

Crews responding to the fire in the facility's basement found the wounded man, who was transported to a hospital, Aurora police said. One other person was treated for smoke inhalation.

There was no explosion and the fire likely spread through the use of an accelerant, police said. – Reuters