CHICAGO, Dec. 24 (Xinhua) -- Dense fog has disrupted the air traffic at Chicago's two international airports, with hundreds of flights being canceled or delayed on Tuesday morning.

The U.S. National Weather Service has issued a hazardous weather outlook for parts of north central Illinois, including the Chicago area, due to "significant fog risk."

The dense fog has caused more than 70 flights to be canceled in the early morning, the Midway International Airport confirmed on its twitter account.

As of 1655 GMT, the average arrival delay at Midway was 56 minutes, while the departure delay 36 minutes, affecting about 85 percent of all scheduled flights, according to flightradar24, a global flight tracking service.

At the O'hare International Airport, 75 percent of all the outgoing flights have suffered an average delay of 33 minutes, while most of the incoming flights gradually coming back to normal, with only a 5-minute delay in average.

The dense fog once forced the U.S. Federal Aviation Administration to ground all flights at Chicago's two main airports in the early hours of Christmas Eve, but the restriction has been lifted later.

"This ground stop at Chicago O'Hare is ruining a lot of people's travel plans today... mine included," tweeted Courtney Spinelli, a local TV reporter.