WASHINGTON, Dec 23 (AFP) - Airline cancellations topped 5,700 U.S. flights on Friday as massive winter storms snarled airport operations around the United States and frustrated tens of thousands of holiday travelers.

That followed nearly 2,700 canceled flights on Thursday, while just over 1,000 flights have already been canceled for Saturday, according to flight tracking website FlightAware.

Passenger railroad Amtrak has canceled dozens of trains through Christmas, disrupting holiday travel for thousands.

Highways in the Midwest faced lengthy delays because of snowy weather or crashes and authorities in parts of Indiana, Michigan, New York and Ohio urged motorists to avoid nonessential travel.

The Federal Aviation Administration (FAA) imposed ground stops or delays for de-icing at a number of U.S. airports because of winter weather.

Transportation Secretary Pete Buttigieg told CNN the U.S. aviation system "is operating under enormous strain" with two different storms and high winds affecting airports around the country. About 10% of U.S. flights were canceled on Thursday, Buttigieg said.

Another 10,400 U.S. flights were delayed on Friday - including more than 40% of those operated by American Airlines (AAL.O), United Airlines (UAL.O), Delta Air Lines (DAL.N) and Southwest Airlines (LUV.N) - after 11,300 flights were delayed Thursday.

Southwest canceled 1,238 flights on Friday, 29% of all its scheduled flights, while Alaska Airlines (ALK.N) canceled 507, or 64%, of its flights.

Seattle-Tacoma International Airport had 357 flights, or 63% of departures, canceled Friday. The FAA lifted a ground stop there due to snow and ice but late Friday delays were still averaging nearly three hours.

Nearly half of departing flights at Detroit Metro were canceled, along with 70% at Portland, 38% at New York's LaGuardia, 29% at Chicago O'Hare and 27% at Boston.

Chicago was facing dangerously cold temperatures with wind chills hitting minus 24 degrees Fahrenheit (minus 31 Celsius).