CHICAGO—An airfield lighting outage has forced the shutdown of more than half of the runways at Chicago's O'Hare International Airport, delaying flights at one of the world's busiest airports.

Chicago Department of Aviation spokeswoman Karen Pride says four of the airport's seven runways were closed for about 40 minutes Thursday night.

She did not know exactly how many flights were disrupted by the outage on the airport's north airfield. The runway closures lasted from about 9:20 to 10 p.m.

The cause is under investigation.