An evacuation following reports of smoke at the Elgin FAA Radar Facility has forced ground stops at O'Hare and Midway.

The FAA started issuing revised flight departure times to airlines Tuesday afternoon after an approximately two-hour “ground stop’’ halted all flights to and from Chicago’s two airports because of smoke in an air traffic radar facility serving northeastern Illinois, airline officials said.

The FAA did not immediately say when full normal operations would resume at its terminal approach radar center in Elgin. The facility handles all commercial flights serving O’Hare International and Midway airports. 

According to FlightStats, which tracks airline operations, 545 flights have been canceled at O'Hare and about 40 at Midway since the ground stop was ordered around 11:30 a.m.  About 850 flights were delayed at O'Hare and 160 at Midway.

American Airlines reported that, as of 1 p.m., it had canceled 34 flights and diverted 22 planes that were already in the air to other airports.

American Eagle, a subsidiary of American that flies regional jets, also canceled 34 flights and diverted 19 planes, American spokeswoman Leslie Scott said.

US Airways, which is merging with American, reported only one cancellation and one diversion.

United customers traveling through O’Hare this afternoon and tonight should expect delays and possible cancellations, United spokeswoman Christen David said. United issued a travel waiver giving customers the option to change their travel plans without incurring extra fees, she said.

The ground stop was ordered as FAA workers were evacuated from the radar facility and operations transferred to the FAA's Chicago Center in Aurora, which usually handles just high-altitude traffic.

The smoke was traced to a faulty ventilation motor and the workers were allowed back into the facility around 1 p.m.

The ground stop means no flights were allowed to depart, and flights bound for Chicago were held at their departure cities.

jhilkevitch@tribune.com | Twitter: @jhilkevitch