Airlines including Delta Air Lines Inc. and American Airlines Group Inc. canceled hundreds of flights early Saturday as an icy winter storm struck southeast U.S. cities.

The National Weather Service warned of hazardous conditions in areas including Atlanta, Delta’s largest hub, and Charlotte, North Carolina, an American hub. As ice and snow piled up, more than 385 flights were canceled at Hartsfield-Jackson Atlanta International Airport and more than 164 at Charlotte Douglas International Airport, according to flight-tracking website

American has already canceled more than 850 flights due to the storm, the airline said, as well as 50 flights scheduled for Sunday. Delta has canceled about 175 flights, primarily arriving at or departing Atlanta, after a layer of ice an inch (2.5 centimeters) thick piled up at the airport, the company said on its website on Saturday morning.

“Ice is worse than snow, since it’s more difficult to de-ice the aircraft,” Delta spokesman Michael Thomas said by phone. “I expect operations to tick up to a normal pace later in the day.”

Airlines had canceled more than 1,900 flights into, out of or within the U.S. as of 2:45 p.m. Saturday, according to FlightAware.