The Chicago-based carrier canceled fewer than 5 percent of its 3,300 flights worldwide on Wednesday, spokeswoman Megan McCarthy said. Those resulted from moving pilots and crews to locations where travelers wanted to get home after the Christmas holiday.

"We're working very hard to get our operations back to a normal schedule," McCarthy said. "Our team has been focused on getting crew and aircraft in the right locations to fly our customers."

United's chapter of the Air Line Pilots Association criticized the carrier for not having enough pilots to fill the gaps. Capt. Mark Bathurst said United's cancelations dwarfed those by rivals American Airlines and Southwest, which operated under similar weather conditions.

"It is unconscionable that United would allow this gaping hole in the schedule during the most critical time of year for holiday travelers and then try to shift the blame to acts of God," Bathurst said in a statement. "This is nothing but further evidence of a leadership team that is simply incapable of managing a world-class airline."

McCarthy said she had not seen the statement and did not have an immediate comment.

Discount airline Skybus also canceled several flights Wednesday. The carrier based in Columbus, Ohio, also suffered a chain of service interruptions on Christmas after one plane that serves multiple cities was reported to have mechanical trouble.

At Detroit Metropolitan Airport in Romulus, fog Wednesday morning forced some incoming flights to be held at their originating airports, but the Federal Aviation Administration reported arrival delays of 15 minutes or less by the afternoon.